Last night I decided to give the power board one more thorough check - still nothing....
So, what the hell I thought....it's going to have to come out anyway, and carefull desoldered and removed the transformer.
And there it was...
One of the tags on the transformer's primaries had snapped inside the board hole, so although the tag looked nice and secure there was no solid connection - in fact I've continuity tested all these tags before to make sure there was a good solder connection to the board. It must have just been resting against the unsnapped part that was soldered to the PCB.
Hence the intermittent cut-outs in the past.
So I installed a flying wire on the transformer tag and ran it to the next solid connection on the board, resoldered the good tags in and at ~3am this morning i finally got it working.
My pessimistic outlook means I'll refrain from calling it 'fixed' for a while, but all the evidence points to this being the problem.
Oh, and I only electrocuted myself once !