Thought I'd split this out of the other thread!
So I got a new BNC cable in the post yesterday and it works, we have RGB on the PVM now so cable is all good.
As The Last Bandit rightly pointed out, the PS2 syncs over green with DVDs. I played a DVD and a nice solid picture magically appeared on the LMD over RGB. (with a tint of green! - changed the input channel option from RGB to component and the colours improved a bit)
So now, the big question, would it be possible to do a bit of cable editing and send regular sync through the green in line for other consoles?
Or is it doomed to only sync with items that already sync on green?