This is an old thread, but for future reference:
Another extremely good source for Leica service is the legendary Malcolm Taylor. He's not in London proper but (at least when Royal Mail is not on strike) he's just as quick.
This courtly gentleman will, after examining your gear, explain to you clearly and in detail what he needs to do to it and why. A chat with him is always very enlightening - and a very pleasant experience. When he is finished, your kit will work just as well as the day it was packed up at the factory.
He's not cheap, but his prices are very reasonable for the quality of his work, which is unsurpassed. And your gear will be back to you very quickly. But not until he has checked it over and serviced it thoroughly. Search about, and you'll find many others as pleased with his service as I am.
He doesn't seem to use e-mail, but you can reach him at
Upper Lye Farm
Aymestrey
Herefordshire
HR6 9SZ
01568 770542