Thanks for the reply. Do you have a youtube channel called "That Tim Guy"? If so, I'm subscribed to the channel, the videos are great.
Anyway, regarding the harness, the way that I tested the board on a RGB scart TV was to use some small solid core wire connected to the scart cable and plugged directly into the wiring harness connector which in turn is connected to the board. So the harness is good as well.
So, I'm out of options. Well almost. I've sent the chassis off over to someone here in the UK who's pretty good with monitors and he should hopefully be able to fix it for a reasonable price.
I'll post a few videos ups on my channel soon once I get around to editing stuff. I also have a load of footage for the Pac-Man build. I think the Pole Position has been distracting me