I removed the last W10 update, KB4565489 and the watermark has disappeared. I can access the activation page but can't contact activation server! No current keys are accepted, including W7 key.
I intend to check and rectify hard drive and leave W10 as it is, since everything appears to work.
Thanks for your help. I may re-open this post when things go wrong!!

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Just to update, I restored an AOMEI Backupper system file from May, and I now have a digital registration with MS.
sfc /scannow still fails at 42%, but hard drive checks out AOK. Weird one huh?