Thanks for the thoughts, Lee. Yes I did clean the battery compartment with my contact "pen", there was a slight trace of corrosion on the inside of the removable cover, fortunately the previous owner had removed the batteries before putting the camera in his display case (where it stayed for 15 years, apparently). This case must have been humidity-controlled - oddly, the plastic shoulder-pad on the neckstrap had dried right out and broken.
I use Newton Ellis & Co. in Liverpool as my regular repair service, the guy I deal with uses an R3 himself, so I'm pretty confident he will nail the problem. I feel the economics add up - it seems easy to find mint-condition
R3s which have been unused for a long time for around £130 ($250), after servicing for around £100 you end up with a Leica in as-new condition for £250. I always feel the ideal with Leica is to find one that has had one or two rolls of film put through it every year, just enough to keep it going, not enough to wear it out, but haven't managed this with R3s!
Regards,
David