Thanks for the thoughts on the switch problem.
I did the diagnostic check and everything seems fine, I think the trouble is definitely in the switch.
I took the panel off and looked around, looks a bit complex in there and we have a gig tommorrow night, so decided discretion was the better part etc: and settled for a squirt of contact cleaner down the sides of the switch.This has made some definite improvement. I will try something further when I have more time before a job. Do you know any place where parts may still be obtained ? It would be nice to have the new part before dismantling if possible.
Thanks again for the assist.

Originally posted by technicsplayer:
sorry, I've not seen this so have no contribution to make. It certainly sounds like contact problem from the description, so the panel has to come off, maybe a squirt of cleaner to the switch may help, or the switch needs replacing? If you switch on holding down F3 and F4, then press Panel Set it proves the SD led and circuits are working, but that cannot diagnose an intermittent mechanical problem with the switch. Good luck.