Thanks Paul,
I rang the UK activation line 08000188354, and after a few automated failures I managed to speak to a real live person. This woman was able to sort out the problem in less than 5 minutes and gave me an activation code which works well enough on Windows 10.
There is an error code when running updates, but I can live with that.
I just don't understand how Microsoft can try to block you installing software which you bought and paid for. Its fine that they no longer support it but don't block users from reloading the software.
Thanks again Paul, problem solved.