Right, update...not looking good for mobo! Took memory cards out, same problem. Unplugged hard drive and DVD drive from SATA sockets on motherboard. Same problem. Unpugged 4 pin 12v cable from motherboard....now fan stays running until mains switched off. At all times, when mains is connected a green LED on motherboard remains lit.
So, to me it looks like the motherboard is indeed embuggerated. PC repair man, here I come.....
Thanks for the questions and suggestions.