Just for completeness, years later, because I saw someone looking at this thread, I'll finish the story here so there's no need to visit the other site.
The mysterious out of date thumbnails were ones that are embedded in the jpeg metadata, nothing to do with the cache.
Just for completeness you can refresh them by opening and saving the image (no need to make any change) with Paint.NET.
Yet another reason to hate W10.
Even this method can't stop thumbnail caching.
The Users\...\appdata\Microsoft\Windows\Explorer folder now has so many other things in there that removing system access to it causes a number of tasks to crash out after a restart.
What's worse, it doesn't seem to stop the system getting in there and doing what the hell it likes cache-wise anyhow.
Tried this on the new laptop and when I found it wouldn't work, I checked on my desktop W10 disk and found it too stuffed full of unwanted thumbnails, despite having been upgraded from W8.1 on which I'd successfully suppressed them.