Sounds like the power supply to the power board is incorrect, I think the bridge rectifier no 3 supplies power may need to be tested.

The photos helped and I know that I was getting 5.2 at the 5v test point and about 12.5 at 12v test point.

where can I get one of those bridge rectifiers? Does the rectifier need to be earthed to the backbox as mine is just hanging there. Might be a clue as to it playing up in the past? Thanks for your help I'd really like to try and nut this out.