If you want belt and braces then you can use software like this to extract your Win 10 key.
Thanks v.much. I'd already got it on my system but didn't realise it offered this,
Microsoft - Windows 8.1 (x64) 00179-60488-xxxxx (Key: xxxxx-xxxxx-xxxxx-xxxxx-xxxxx)
Now to decide about whether to go ssd, hybrid or standard hmmmm...