It was when he was putting the shopping away that it occurred to John that he really ought to do something about trying to housetrain Sherlock.

It wasn't that there was any exceptional new outrage. If anything, there were marginally fewer human body parts in the freezer than he found on the average day. But, the routine nature of the situation was rather the point. He realised that he was becoming immune to Sherlock. It no longer shocked him to find jars of anthrax alongside the jam and the tomato ketchup or venomous snakes in the laundry basket. When he found a severed hand in a freezer drawer, he didn't even bat an eyelid anymore – he just stacked the frozen peas or the individual cheesecakes on top of it.

He realised that if he carried on like this, he would end up making more and more accommodations to his strange flatmate. He couldn't keep living his life around Sherlock's eccentricities. It was time to fight back, time to reassert his claim to his share of the territory. He needed to start calling some of the shots.

He started by fixing a padlock to his underwear drawer, so that Sherlock could no longer simply help himself to his socks and pants when he ran out of clean ones himself (which was all the time, as the world's only consulting detective didn't yet appear to have deduced the cause-effect relationship between operating a washing machine and having clean laundry).

Emboldened by this manly display of basic DIY, he next decided to tackle the dining table. He moved all Sherlock's stuff off it and placed it in a cardboard box, which he left on the threshold of Sherlock's room. He vowed that he would keep the table a clear space from now on. Any new clutter which accumulated there would be unceremoniously moved.

Finally, he reorganised the contents of the freezer. He moved anything which looked human to the lower shelves and anything which looked vaguely edible to the higher ones. Then he bought some sticky labels from a nearby stationer's and neatly labelled the drawers either "Food" or "Experiments".

Satisfied with the fruits of his labours, he decided that would do for today. Tomorrow he would put into operation Phase 2 of his masterplan to housetrain Sherlock.

But things did not go quite as planned. When he woke up the next morning, the first thing he saw was the padlock hanging from the chest of drawers unclasped. Sherlock must have come into his room and picked the lock with a hairpin while he was sleeping.

And, sure enough, when he went into the living room, he found his flatmate lounging on the sofa, typing furiously into John's laptop, a pair of John's grey socks on his feet, and when he shifted his position his silk dressing gown gaped open a little bit, flashing a glimpse of what John was fairly sure were his own Y-fronts. The table which he'd laboriously cleared the previous night was also now covered with twice as much clutter as had been on it before.

He opened his mouth to yell at Sherlock, but stopped himself. There was no point. Getting angry with Sherlock just gave him the upper hand, because you put yourself in a position where you were irrational and vulnerable, while the other man always remained infuriatingly, unnaturally impassive. Best tackle this when he'd calmed down a bit. Perhaps he'd feel more serene when he'd had some breakfast.

He grabbed what he assumed were frozen mini croissants from the freezer and stuck them in the oven. It was only when he had heated them up that he realised that he'd just cooked a plate of human ears. He checked the freezer again – Sherlock had come and jumbled it up during the night.

White-lipped with anger, he stormed into the living room.

"Sherlock! You broke into my belongings during the night, you messed up the table and the freezer which I spent hours tidying yesterday, all for no obvious reason other than just to piss me off. Now what the hell is going on?"

"You could have asked first," snapped Sherlock, waspishly, not bothering to look up from the screen.

John actually started to feel guilty, before it occurred to him that, while Sherlock might have a point vis-à-vis the dining table or the freezer, he was damned if he saw why he should have to ask permission before taking appropriate steps to protect his own underwear.

Even so, he found himself apologising. Sort of.

He took a deep breath. "Look, Sherlock, I'm sorry. Yes, I probably should have talked to you about it first. But you've got to understand that you do kind of take over the flat a bit. I think you sometimes forget that I live here, too."

Undeterred, he decided to try again a few days later, this time taking a slightly different tack. He'd been planning to spend the weekend at Sarah's, as he usually did. But then he started to think, why the hell should he? Wasn't that another symptom of his continual efforts to walk on eggshells around Sherlock? Why couldn't he have his own girlfriend over to his own flat of a weekend if he wanted to?

Of course, he could hardly expect Sherlock to go out for the evening, just because Sarah would be there, or shut himself in his own room for several hours. That would be unfair. But maybe the three of them could do something together? Well, no, perhaps that might be a bit odd. Maybe if they asked some other people over, too? Made it more of a social event?

And that's when he hit on an idea which he hoped would not only help reassert his right to the shared space, but also might help to socialise Sherlock a bit. Get him to loosen up. Give him some practice in behaving appropriately around other people.

"I was thinking….," he began, tentatively, next time he saw his flatmate. "Maybe I could have Sarah over here this weekend? It would make a change."

"You must do as you wish," muttered Sherlock, stiffly. "You live here, too, as you're always reminding me."

"I was also thinking….maybe we could invite Molly over, as well? Have a…" he took a deep breath, as he plucked up the courage to come out with it. "Have a dinner party?"

"A dinner party?" said Sherlock, arching a brow suspiciously.

"A dinner party, yes. It's a social event where people who don't normally live with you come to your flat and a meal is generally served…"

"Yes, I do know what a dinner party is! Are you asking me to make myself scarce that evening? Is this what this is about?"

"No, Sherlock! No, not at all! I want you to be there, too. I want us to do this together."

His flatmate looked a little shocked by this suggestion, but he didn't immediately protest, so John decided to proceed with the rest of his idea.

"I'd take care of the main course. I was planning to do this spicy chicken dish I picked up the recipe for from Abdullah, an Afghan doctor I worked with in Kandahar. I've made it before and it's pretty nice. I'll get some wine in, too. I thought I'd ask the girls if they'd mind bringing puddings and…" He paused, uncertainly. "And I was wondering if you'd be able to make a starter?"

There was a brief silence. Then, to John's surprise, Sherlock said, "OK."

"OK?" asked John, incredulously. Had it really been that simple?

"Yes, OK. I can do that. Right…um…yes. You're doing an Afghan dish, you say? I think I'll do something Moroccan. I imagine that would go pretty well with that."

Sherlock's voice had taken on that overbright tone that it normally did when he was blagging, but John was so amazed that he'd agreed to it at all, that he took no notice of that.

After that, things seemed to continue to go according to plan. He broached the subject with Sarah and, although she didn't look best pleased, she agreed to come and to bring a cheesecake. Deep down, John knew there was little love lost between Sarah and Sherlock. He had no idea why – personality clash, perhaps? But he thought that the two of them could probably survive a few hours in the same room together without their mutual antipathy breaking forth into open warfare.

Molly was so pathetically excited and grateful to be invited, that John felt he had to sit her down and emphasise to her in words of no more than one syllable that Sherlock had asked her just as a friend, that she shouldn't read too much into this. But he was relieved that he would have another person there to act as a buffer between Sherlock and Sarah and he began to have high hopes that this dinner might make some inroads into the task of civilising his unruly flatmate.

As the day approached, however, John became increasingly nervous and started to ask himself if this had been such a good idea, after all. He'd never really expected Sherlock to make a competent job of the starter, but his main concern had been that his flatmate would find the whole thing deeply boring and not pull his weight. He'd been quite prepared for a situation in which Sherlock did a tokenistic, rush job, like plonking two lettuce leaves and a cherry tomato on a plate and calling that a starter, or opening a can or plastic tub of readymade soup from the supermarket. Or a situation in which he cheated and delegated the whole thing to Mrs Hudson. Or just didn't bother with producing a starter at all. Or just didn't turn up.

Any of these John could have dealt with, worked round and taken in his stride (although he wasn't quite sure how he would have coped with a tearful and heartbroken Molly if the latter happened). But the one thing he had not been prepared for, and which now filled him with a deep sense of foreboding and dread, was Sherlock taking the whole thing deadly, deadly seriously. That was the trouble with him – he basically only came with two settings: contemptuously uninterested or manically, obsessively focused. And unexpectedly, quite bizarrely, he had gone into the latter mode over this dinner party, approaching the task with the dedication of an athlete training for the Olympics.

John on more than one occasion caught him sneaking into the flat at unusual hours, clutching a plastic carrier bag. When he saw he'd been clocked, he'd blush and say, "Ingredients for my starter. Can't show you. It's a secret," before disappearing into the bowels of his room.

The night before the dinner party, John went into the kitchen to put the chicken for his own dish in to marinade and he found a bowl on one of the work surfaces, covered with a plate. He was about to lift the plate up to have a look at what was inside, when Sherlock came rushing into the room, very agitated: "Don't look! It's for tomorrow! It's supposed to be a surprise!" Giving John a filthy look, he snatched up the bowl, tucked it under his arm and swept off to his bedroom with it. A few seconds later, John heard the scraping of a key in the lock and sighed – Sherlock needn't have bothered locking the door, as HE, at least, had some concept of privacy and respect for other people's boundaries.

The amount of effort Sherlock was putting into this was baffling. It almost seemed like he was trying to impress him, but that, surely, couldn't be the case, could it? Sherlock never tried to impress anybody – he didn't care enough about anyone to be the least bit interested in what they thought of him (well, OK, with the possible exception of Moriarty. Because Sherlock considered Moriarty almost his intellectual equal, competitive testosterone kicked in, and he considered it a point of pride to prove to the slimy little Irishman that he could not be outwitted. But not ordinary, fallible mortals like colleagues, flatmates, friends).

But it wasn't just baffling, it was also deeply troubling. Because if Sherlock had actually made an effort, it seemed likely that he would also actually care whether John and the guests ate the results or not, something which John had not foreseen, and he was becoming increasingly worried that whatever Sherlock produced would not be fit for human consumption. If they refused to eat it or discreetly emptied the contents of their plate into a plant pot, there was no way of knowing exactly how Sherlock would react. The sulking could go on for weeks.

However, the day finally arrived and John did his best to put his doubts out of his mind. He spent most of the afternoon preparing the chicken, vacating the kitchen so that Sherlock could work on his top secret dish an hour before the guests were due to arrive.

Molly turned up first, five minutes before the stated start time. She looked stunning. She was wearing a beautiful silk dress, printed with a delicate floral pattern in soft pink and cerise, had spent ages curling and arranging her hair and had done her make-up immaculately. Decked up like that, she would definitely turn heads in a room full of normal men.

Unfortunately, though, the objects of Molly's affection were never normal men and John reflected, sadly, that the only way she would manage to get Sherlock interested in her appearance was if she had a bullet hole in her chest, strangulation marks on her throat and half her face ripped off by a specially-trained man-eating Rottweiler.

Still, he wasn't working on a case at the moment, so there was some chance that he might eat some of the delicious-looking homemade pavlova she was clutching. If he did, that would probably be enough to make her evening.

Sarah turned up not long after, looking far less groomed than Molly, but just as lovely. John couldn't resist sneaking a quick snog, although he did his best not to spoon too ostentatiously in front of Molly. It wouldn't be tactful.

He poured the girls and himself a glass of wine each and suggested that maybe they should sit at the table. Sherlock had been working on the first course for an hour and a half now. It must be ready soon, surely?

It wasn't. There was still quite a long wait. But, eventually, just as John was thinking of checking to see if his flatmate had passed out, Sherlock emerged from the kitchen, a look of intense concentration on his face, as he tried to manoeuvre his way to the table balancing four small plates of what, to John's untrained eye, looked like cat sick – a yellowish-brown, amorphous gloop which had been plonked on the plates with marginally less finesse than a dog deposits its mess on a pavement.

Still, it didn't smell like cat sick. In fact, to John's surprise, the aromatic scent of North African spices and citrus that wafted up from the plates was actually quite inviting.

Sherlock dropped a plate ungraciously in front of every guest, before retreating to his own chair. There was an awkward moment of silence, as Sarah stared down at her plate with an undisguised look of horror etched on her features and Molly fiddled with her cutlery nervously, an uncertain half-smile on her face. She opened her mouth, presumably to trot out some conventional platitude such as "You've gone to so much trouble!" or "This looks nice!", but then closed it again, having thought better of it. She always had been a really unconvincing liar.

But Sherlock didn't seem to have noticed the effect that the sight of his culinary efforts had had on their guests. He had become distracted by a scientific journal which he had found under his cushion and had not been able to resist the attractions of an article on case histories of the use of mitochondrial DNA in South American murder investigations. But eventually he did seem to pick up on the fact that no-one was eating.

"Well, come on, everybody! Dive in! You don't need to wait for me!"

John's natural good manners and his military training persuaded him that he should be brave and go first, in order to protect the womenfolk from what might be an unpleasant ordeal. With trepidation, he picked up his fork, speared a chunk of the indeterminate mass that had been set before him and put it in his mouth. He began to chew mechanically.

He spluttered, burst into a coughing fit and accidentally spat about half of his mouthful into Molly's lap.

The women both looked at him, concerned. Sarah quickly rushed to pour him a glass of water and started to clear the plates away, assuming that the starter was completely inedible.

But John put out a hand to stop her and hung onto his plate for grim death. He hadn't been coughing with disgust or because the food was so indigestible. He'd been coughing with surprise.

"Sherlock, this is FANTASTIC!" he exclaimed, helping himself to another big forkful and savouring the heavily spiced, sweet and sour taste.

His flatmate made no response and still looked like he was deeply absorbed in Forensic Pathology Quarterly, but a faint redness at the tips of his ears betrayed to John that he had heard the compliment and was not completely immune to it.

Sarah and Molly shot John inquisitive glances, but he discreetly nodded, pointed to his plate, and gave them the thumbs-up. Puzzled, they picked up their own forks, and made dainty, tentative forays into the North Face of the food. They, also, responded with a mixture of stunned surprise and utter delight to Sherlock's unusual starter. Sarah looked at Sherlock with begrudging respect. Molly gazed at him with naked adoration – was there anything this man couldn't do?

John just concentrated on enjoying his food. What was in it? He picked up a mix of tastes and textures – the crumbliness of cooked chick peas, the crunch of sliced almonds, the sweetness and chewiness of dried apricots, the smokiness of cinnamon, but all dominated by the pungent, bittersweet taste of….was it preserved lemon?

"Sherlock," he asked, barely audible, as he had his mouth full, "what is this? It's incredible."

Sherlock, still not lifting his eyes from the page, muttered, matter-of-factly, "A lemon entrée, my dear John."