Ok, problem solved. I obtained a replacement power supply board (G3), received it today and fitted it. Nada...exact same symptom, flashing error 5.
After a few well chosen sweary words, I decided that I must have missed something, or less likely, fitted a faulty replacement board. After checking all the main supplies again, and finding nothing that jumped out at me, I started working my way back towards the power supply from the main control board.
Now I noted right from the off that some of the aux supply rails were missing, but as these come from the regulator board (G2) and have inbuilt shutdown facilities, I figured that they would only come up when the rest of the set had self tested etc. However, tonight I decided to look further into what actually switched the regulators on. This being the PWR_SW line from my reading of the schematic. Low and behold, I find a dead short on it to ground. No wonder there was no logic signal turning on the regulators for the aux supply rails. After a quick look at the schematic, I couldn't see anything obvious that could actually cause this, I checked and double checked the spare boards that I have and still couldn't see how this was possible, so it was divide and conquer time. I removed all the plugs from each board and checked again, still shorted to ground. Then I realised that the main control board and main A/V board were still connected by the flex ribbon cable. I removed it and no more short, on either board. Looking at the ribbon cable, I couldn't believe my eyes. I can only guess that the last so called engineer had no clue how the connector system works and had forced the ribbon back into the sockets. at least half the tongues were torn up and all bunched and lapped together on the main A/V side. Luckily, I had gotten a replacement ribbon with the main control board that I obtained, so after fitting it no more shorts. Voila, the set bursts into life and I'm greeted with a cracking picture. I was half expecting the replacement control board to be damaged, but it survived. This has to be a first for me to get caught out by a mangled flex ribbon rather than finding one making poor contact. but there you go, mystery solved
I'm happy it's working...time to re-sell the spare boards...