OK, here are the people I would use in the UK.
First Tony Allgood @ www.oakleysound.com
Telephone him on 01768 881934
I have used him for years, in that time he has repaired the following for me....
Oberheim OB1, Minimoog (Total rebuild), Prophet VS, Korg Mono/Poly, EDP Wasp, RSF Kobol Expander, Oberheim SEM and of course built my big Modular. I'm sure there were others!
I know he has repaired LOTS more and his attention to detail is fantastic. Without doubt the best in my opinion.
Then there is James Walker @ www.synthrepairservices.com
Telephone him on 01782 262611
While I have not used James direct yet, I have had lots of gear that was serviced by him from when I got stuff from Music Control. Again a nice guy and one who does a great job.
There is also Kent Spong at KSR, he can be reached on 020 8873 2044. I had my Jupiter 8 repaired by him and he does most of the restoration work for RL Music. However I would may be not use him for Servicing or Electronics work, more for the cosmetic side of things.
Some of the other tech's in the UK I could tell you horror stories about if you wanted, but I have tried never to deal with them.
In the US I have had dealings with Kevin Lightner at www.synthfool.com
and he was first class, however he tends to be booked up alot and does not service everything anymore, just the top notch vintage stuff.
I am also using Phil Cirocco of CMS, at the moment to get my ARP 2600 working again. www.discretesynthesizers.com/arptronics/Arptronics.htm
Whilst I have not used him before he is the best ARP tech on the planet.