Thanks A_Camera, I will look at the Windows repair tool from Microsoft.
i said 'it's a known cause' only because my searching suggested that there have been many instances of programs crashing in general due to problems with this file, not that it's known for UCCNC. The CNCdrive forum has only one reference to a crash due to that .dll file and it relates to using later versions of the software (1.2112 and 1.2113).
I think I have a clone HDD for this laptop from about 18 months ago but I will have to search for it (I've moved house since then!) which might be an interesting test. I certainly have a spare mechanical drive I can load a new installation of W10 onto. I'm not going to trash the current installation if I can avoid it.

Kit