I have recently had my 3.5F fully serviced by Newton Ellis & Co and they have made a splendid job of it. The taking lens is now spotless, shutter speeds are all buzzing away accurately and the rough patch when turning the focussing knob has gone. I also never knew how smooth the film advance should be...it's like silk now!
They were not expensive (especially for a proper repair shop) and were very quick. I mailed the camera to them on a Tuesday and received it back, fully serviced the following Wednesday.
A very positive experience and they will definately be my first port of call if I need a camera servicing again.
http://www.newtonellis.co.uk/