Nick Gregory G0HIK
Thanks all for the helpful comments.
I've just got it licked, well it looks like it anyway.
It was an overload on the +/- 15v rails. I thought I had disconnected every consumer of the +/-15v from the motherboard, but hidden under the YIG filter was a supply to the 100MHz L.O. Low and behold there were two +/- 12v regulators, decoupled by Tants. One was S/C but both changed out of course.
Several lessons learnt, every day is a school day, I just need to try and retain it all.