After serious thinking John Rumm wrote :
For future reference. if you have you PC logged in with an online account
(MS, outlook, live etc), then it can link the windows activation to the
account. Hence you can change a mobo, and when you login again it will
recover the activation state.

(which sounds like what happened here - even if not on the first attempt!)

I just didn't manage to log in at all, until it actually got as far as
being fully booted up into Windows. It seemed to take pity on my
predicament :-)