Been doing more digging on this and came up with two handy apps from MSoft which are on the Store.

(1)WINDBG Preview ...a WIN DEBUG tool that reads and analyses dump files. Not the most intuitive GUI but handles all dump files I could throw at it. It did not tell me much about the present problem which I did not already know from other sources but had I had it to start I would have got there faster.

(2) DIAGNOSTIC DATA VIEWER ...displays diagnostic messages stored in Win 10. Threw up messages I had not seen before, most notably that every time I ran the 2004 update there was a corresponding error message related to DISM.EXE. Since this features on MSoft's list of known problems I now suspect that my problem in installing the 2004 update is directly related to this. I had not previously appreciated that DISM runs in the background during the final stages of an update, but this demonstrates clearly that it does. Should know soon whether this is the root of the problem.