Hi Guys
Didn't have a plastic screwdriver but I turned everything off, backed off the voltage screw, which only turned about an 1/8th of a turn anyway before it was seated.
Fired the machine back up and I now have 5.09 volts at the +5v terminal.
Good enough I figure, still doesn't run properly though.
Did a bit more research on Jamma, I suspect the board is Jamma 'cos it has a 28pin connector, the  cabinet has an 18 pin connector and there is an adaptor loom fitted in between to hook the two together.
I'm really not getting too far with this machine, I think I will sit back and think about it, but I am progressing.
From other research it seems as though a possible cause is poor contact between the eproms, already addresses but perhaps need to pull all the eproms and clean the legs, a delicate operation so still pondering on that one.
Anyway, thanks so far for all the excellent input.
Cheers