Hey, I thought I'd give an update on this. Took me a long time to find time to sort it out.
Sure protector was dead. Socket had to be replaced.
I disconnected all the cables from the PSU so it was just the supply with no load. Plugged it in and it instantly tripped the power. Didn't even need to trick it into staring up. Unit was only bought 10 months ago. Got a refund.
Just put an RM850e into the machine. All seems to be working fine.
Fingers crossed just a dodgy PSU all along.