Well, I took it apart again, taking the fan/heatpipe assembly off and didn't manage to find anything. Did pull out a ball of lint about the size of an almond, which was probably the source of the death of whatever component has died.

Everything looks as good as can be, no easily identifiable burn marks or anything. Still the same symptoms of almost starting up when it's been left off for a long time, then refusing to do anything but blink the lights when you press the power button after that. Sometimes it blinks them for longer than others, but the more often you do it, the shorter they stay on.

Does this sound like a capacitor or some other component that's dieing as soon as it heats up? Is there any way to diagnose which one it might be?

The only thing left that I could swap out is the processor, which I may try tomorrow. Is there any chance it could be the processor itself?

I'm open to any advice... (Though I think I'm off to find a stiff drink at the moment, so there's no need to reccomend that)

Matthew