Three suggestions, particularly if you're in or near the London area.
First, I would definitely recommend calling up Paul Roach of the former
High Street Radio and Photographic in Croydon. Sadly, his father, the legendary Reg Roach, passed away last year and the shop no longer exists in brick-and-mortar form--a true loss to Leicaphiles in London--but Paul actively buys and sells online (and he'll be happy to meet with you to examine your gear and work out a deal). Very trustworthy gentleman, I've bought many things (M3, M6, Hasselblad, Rolleiflex, etc., etc.) over nearly two decades from him, and have sold/traded things with him and his late father on many occasions.
I'd actually, given my long relationship with him and personal knowledge of his business practices and expertise, call Paul first if I were selling something myself (and in fact, he's got a 90mm Version II Summicron of mine now that he's offering).
I can also highly recommend
Aperture. I've bought a number of things from them, although mostly in terms of film and processing, and they're highly expert, honest, and reasonable. They have three branches, the shop on Riding House Street is where they handle Leica gear.
If that doesn't work out, you might also try
Red Dot Cameras in London. They have a very good reputation as well, although I've never traded with them as I was always able, when it came to Leica gear, to find a much better deal on the things I wanted from Reg and Paul.