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Excel/Win 10 thing again.
Got back yesterday; had a good bash at this issue. It seems to me that, not only to MS have the arrogance to ignore the settings of the OS it was replacing, and setting things as if the whole world were Americans; it appears to me they have the same setting in more than one place. I had already changed some settings from US to UK, but I found a further one last night. Not easy, as control panel access is not obvious as it used to be in the previous versions. I ended up finding a link to it on some window that I finally came across. I've only just had a quick look at the spreadsheet that was getting messed up, and it seems corrected.
As for Nvlddmkm, I'll get back to that soon.