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A new cabled USB keyboard didn't help. Still no response to keypresses. Nor did plugging in the newly created Win10 boot flash drive. Not that I've tried it on a working PC.
(I gave up trying to download the Ultimate Boot CD iso. The home site only had a torrent download option, which I'm uneasy trying to arrange on someone else's laptop, or mirror sites that didn't seem to have it, or, the one that I eventually tried and found it on, the antivirus blocked because it reckoned it contained something it didn't like.)
All so frustrating.