OB ----- thanks for removing my Product Key ---- I did not know this was an issue, I wont do it again!
USAMASA --- I got the "This machine is permanently activated" response, so that is OK..
Regarding the 'charge for updating', although in my annoyance I accused the tech of a 'scam' it was not meant literally. I am confident that I was communicating with a genuine MS advisor, no malware corruption, etc. I was not on the phone, I was engaged in a Chat in MS Support. There may have been a misunderstanding. When I was asked for my Product Key I replied that I could not find it (at short notice) as it is not in Settings, Control Panel or anywhere else I tried. I now think this was taken to mean I had not got one, and the fee is for someone without a Key. After leaving the Chat I found the Key I quoted earlier (now removed) with Speccy and with Belarc Advisor. As the Key is now confirmed (thanks for the link) I will probably try Chat again tomorrow, drawing attention to the misunderstanding.
Two things I find unhelpful about MS Support is that every (without exaggeration) tech / advisor tells me something different, very often contradicting each other, plus they do not seem to be well informed about issues involving small devices.
I simply cannot create any more free space on my SSD. Absolutely everything that can be moved or deleted has been. All that is on C is the OS. Unless of course there is something I cannot see. Invisible files should not be removed anyway, should they? The techs have only found empty folders. Also I assume a clean re-install would remove anything unnecessary, lurking unseen?