You could try Behind Bars in Kenilworth. Andy Spencer the owner is trustworthy and competent. I'm not sure he's a K expert, but he's certainly worked on plenty of BMWs, so there shouldn't be any surprises in a K.
The trouble with K valve checks is that it's inherently expensive because of the time to strip the bodywork down.
You almost want to do a deal where you strip it, push it into workshop, he does the valves, then push it out and stick all the plastic back on. Probably wouldn't go down well with most indies though.