does the socket work?
check fuse in plug check plug wiring (if not moulded on)
switches on rear of PSU if fitted make sure set to correct voltage and in on position.
If certain PSU is good: disconnect all drives and see if it powers on to and error massage.
if still dead remove memory and see if it powers on and beeps with error codes (m/board speaker must be fitted or leds on motherboard to give code No.
if still dead then looking like bad motherboard or cpu remove from case lay on paper and retry (checks for shorting to case)
Take PSU back to Maplins and ask for a refund