Hello again. PSU is fine (voltages all stable, no obstruction to vents, fan working OK), the machine does switch on when the power button is operated, the HD spins up, the lights come on, the CD drives start up, the fans all start running.
The Power On Self Test (POST) doesn't run, and nothing else happens, screen stays blank.
Checked all connections were secure. Have removed, inspected and reseated:
Memory cards
Graphics card
Sound card
Processor
Got my IT dept to check it out today, they found the same as me. They were able to connect the HD to another spare PC from which I was able to retrieve some data...but not all as the hard drive isn't very healthy either (bad sectors, some files not readable etc). Not looking good for the motherboard/processor.
So, repair or get a new better spec PC for half the price I paid for my original one?!
Thanks for the replies.