Written from Sherlock's POV, in the slightly gonzo cracky way of Hunter S. Thompson's 'Fear and Loathing in Las Vegas,' and with the following liberties taken:
John and Sherlock taking the Rover back instead of the train referenced in the episode, and yes, slashy slash established relationship. Assume certain amounts of OOCness, for a purpose - a lesson for Sherlock.

Sherlock:

We were about 15 kilometres past Exeter on the M5 when the hallucinogens began to kick in again. I distinctly recall saying something in a wavering tone like, "I feel a bit lightheaded, John; maybe you should drive…."

Suddenly there was terrible howling surrounding us and the motorway was full of dogs, all baying and snarling and humping the fenders and leaping onto the bonnet of the Rover, which was going about a hundred kilometres an hour to civilization and London. And a voice was squealing, "Jesus fuck not again! What are these benighted canines!"

Then it was suddenly quiet again. My doctor had taken his coat and jumper off and was tilting back his seat in order to either nap or display himself, the brazen bastard. He knew what that did to me. "You say something, Sherlock?" he murmured, adjusting the aviator glasses he'd picked up in the B&B gift shop. I looked at him suspiciously. He shifted slightly in the seat, the fabric of his jeans taut across his lap. He licked his lips.

"Never mind," I said. "It's your turn to drive." I hit the brakes and aimed the Great Black Beast toward the exit just coming up, never mind it was in the middle of nowhere. "Also," I went on grimly, ignoring the growl emanating from the rear of the Rover and trying not to flinch away from the imaginary exhalations of the beast standing just behind my seat and breathing in my ear, "I think we need to have a little talk, John. About the cup of coffee you got me as take-away this morning before we left?"

"You said the drug wasn't in the sugar," said John.

"At Henry's house," I said. "But you know I like sugar in my coffee. Of course. Stupid me. I thought the grittiness was due to coffee grounds. Residue scraped from the rocks in the hollow?"

The quiver at the corners of John's mouth gave him away; I cursed bitterly and spun the wheel so the Rover bounced off the road and down a farmer's lane. "Fine. You win. I apologize, I am SORRY I tried to drug you and that I terrified you deliberately. And I hope you are satisfied, because now you'll have to hold back the hallucinations with your manly arms and big gun and amazing sexual skills."

I turned off the ignition with a vicious twist, threw off the seat belt and leaned over the gear shift to grasp the edges of John's seat, trapping him. "So, Doctor. Are you ready?" I spoke through clenched teeth. "Because if you don't fuck me immediately, there's this hound just behind me that is more than willing to take my bestiality virginity."

The quiver expanded into a full smile as he tilted up the aviators. "Payback is a bitch, isn't it, Sherlock? Come here, big man. I'll save you."

And with that he grasped my collar and pulled me on top of him.

-the end. woof woof.-

OK, it is silly, but by god, if I had been John, I would have planted a shoe up Sherlock's arse so hard for his little trick... Episode John takes the drugging thing too much in stride. Had to redress it a little for my own peace of mind.