Problem solved
I decided to scrap the PC so I removed all the bits I thought I might be able to use, ie power supply, memory, hard drive, CMOS battery, DVD rom, as I had some time on my hands I thought I'd reassemble the bits, lo and behold when I switched it on, the PC started to boot up and when I shut it down it shut itself down.
Must have been a bad connection on one of the leads, anyway it now seems OK
Thanks for your help