Well it's good news here.
Cutting a long story short, I eneded up phoning MS (amazing really as it was the middle of the night at the weekend)
and going through the manual validation process. They were slightly bemused to hear that XP was already up and running, but eventually gave me the green light at their end.
I did attempt the registry fix first, but got a bit lost at 'change one digit to deactivate Windows' - which one digit?
Anyway alls well and all that...
Thanks to all who helped.
Now it's off to validate Office - Hi Ho and all that