Not a first hand recommendation as I've not had mine serviced yet, but I was recommended aperture on rathbone place for my iii.
They charge £120 for leica service & £200 for shutter replacement.
Red dot are loads cheaper though... £80 for a service.
Was gonna try red dot, as the camera was totally fine until now (although my granddad had mentioned a sticky shutter). Hopefully doesn't need anything special done to it.
Best uk place for Leica servicing? Just got a 70% blank roll out the old Leica iiif :-(