Ah ha, OK thanks, I'm getting somewhere!
So I'm looking at downloading W10 to a DVD/USB from the link
here.
It says:
Is that the Windows 8.1 key I already have? Or when I upgrade to 10 does it give me a product key? Or does it generate a key based on the hardware in the machine (I've read something to that effect).
I will be switching off all the sharing with Microsoft stuff as per the RockPaperShotgun advice
here.
Will that mean I can still get access to a key if I'm not using an online Microsoft account?
Yes, I really am trying to account for any possible problem before I upgrade!
Thanks!