After a long an arduous few hours of almost totally dismantling the 500 just to replace the hard disk and putting it back together it worked. Briefly.

The problem now is the screen just cut out. Tilting it a bit makes it flash back again but it won't stay. Must be the inverter cable as I can see the image faintly onscreen. Great. I don't know if I can face having to take this thing apart again, they're an absolute nightmare. What the hell were Apple thinking when they designed these.

And after a reboot it's working again. I think the battery might be messing it up. Have taken it out for now to see how it goes.