Thanks for the reply, was going slightly mad...
Right, connected it to a wall wart, forced 3 failed boots (as per wiki). on 4th boot, green light for a few seconds, then a single red LED flash before the LED went blank.
I left it for 6 minutes or so, no flashing green LED, no LED at all. BB was warm to the touch at this point.
Disconnected it; upon reconnection, no LED's at all.
Taken outside to cool down a little (yeah, I know the CPU is rated to higher temps).
When reconnected to wall wart, normal boot and flashing blue light.
So I've reconnected to the computer and get mass storage access for maybe 5 minutes, before disconnect. Device then unresponsive until being left for 10 minutes.
Then attempted to force another Firmware Recovery - red LED flashes 5 times before stopping, no green LED even after 5 minutes.
I think I'm at tech support stage; can't keep it stable for longer than 5 minutes, even under no load.