Soldato
Spoke too soon - Win 10 has started complaining about being activated (which is odd given it was fine before) and the troubleshooter isn't working.
Had to find and enter my old WIn 7 licence key in the end, even though I followed the instructions to link it to my MS account and the MS account clearly had it linked!
Was your Win7 key Retail or OEM ? If it was OEM then it will stop working at some point anyway. You can't migrate any Win OEM key to a completely different system, all you can do is update parts of that system.