I'm late to this thread. However, reading through here are some of my experiences:
Optical Instruments (Balham) - I've used this company for polishing / re-coating lenses and they did a superb job at a very reasonable cost. Others have commented on the length of time it takes, but I believe this is because they try to keep prices down by batch-coating. Thus if your lens happens to go in just after a batch has been done, you could face a bit of a wait. Having said that though, I spoke to a couple of top Leica repairers who couldn't believe the low price they charged me for repair!
Malcolm Taylor - Repaired Leicas for me that others have been unable to. Fantastic service, can't recommend him highly enough.