I seem to have resolved the issue, I started to think about heat as the cause, and removed the CPU fan, the thermal paste was almost solid.
I replaced with Arctic Silver 5 as per other posts on this forum, Windows then restarted instantly and (almost) normally.
Once in windows I realised a memory module was not working which I have now replaced, and so far today the system has run just fine - although it's only been on for about 3 hours.
I am 1/. surprised that the thermal paste only lasted about two years from new, and 2/. that heat build up occurred fast enough even from a stone cold startup to cause a shutdown, so I will continue to use the computer and see how we go as can't see anything else wrong.
One last question, what would be an average CPU temp when in low usage - ie: almost idling ?? mine seems to be high - at 64c.. does that sound right ?.. overall system says it's at 37c .. both those reported by Pheonix BIOS.
Thanks for the help.