It's not BBC!Sherlock, but I don't know what verse it was in. I think Moriarty was called "Professor Moriarty", but I'm not sure about that, either.
What I do remember is that Watson was at Moriarty's place. He was there for a longer time, I think, maybe kidnapped? I'm so fuzzy on the details, I'm sorry.( -- cut for explicit description -- )
I'd appreciate some help in locating this fic, because this is driving me crazy. If you could tell me what verse this is set in and where to look, that would already be a big help.
FOUND! All my thanks, forever, go to the lovely and massively talented sheila_snow for finding (and writing) On the Orbits of Asteroids