Firstly a thanks to Martin & David. Yes I have a copy of the service manual and in between other jobs am reading up studiously
I am also well aware of the voltages racing around inside this tester and am very careful.
Anyway I have, not surprisingly, discovered another problem - the resistor R36 has an overwind of wire to make up 200 ohms and one end of this was dry jointed. My copy of the manual shows R36 as 200 ohms (and no tolerance stated) but I have found that other versions of the service manual show it as 200 ohms +/- 0.5%. This one measures 198.1 ohms inc. overwind so I'll replace the overwind with some suitable guage enamel wire sometime today.
I've also found the missive written jointly by Martin & Euan Mckenzie on the VCM's and this is way better than any of the Avo manuals so am confident that a fix is getting close. Another day-another challenge! Mike