aha!

It appears it does actually work, yay! The adapter I was using before had a max current on 300mA and I just noticed on another site selling the screens that they had an option to also purchase a 1200mA adapter with it. So I had a rumage around and noticed that the adapter for my sub is a 12v DC 1700mA, popped that into the screen and it works like a dream

LOL I feel so stupid
It's a very nice screen anyway, bit hard to read at 800x600 or 1024x768 though so I'm using 640x480 and it doesn't fit a 7" very well as there's about 3/4 of an inch wasted at the sides. I'm really pleased with it anyway, playback of DivX on it is amazing too, highly recommended!