Thanks. Yes to Repair and Yes to all compatability options. Also used Repair option from the original CD. Not yet tried completely unistalling PSP8 and reinstalling, partly because of the subsequent tasks of restoring around 30 years of settings,, but also because I'm pessimistic of that fixing it.
EDIT Wed 7 Aug 2024 16:00
Mfc42.dll is apparently at ther root of the issue according to Event Viewer. As I'm at the desperate stage, I downloaded the latest version of that file. It's version ...8064 and 1366 KB in size. I then copied it to replace the orginal (version 8063, 1423 KB) in \System32. No fix.
And I don't understand why, immediately after trying to run PSP8, the three events that are then displayed in Event Viewer still show version 8063, not 8064 as expected! Apart from that puzzle, ikt now has me hesitating about changing it back again...
Also, to add further to my confusion, I note there are several other locations of mfc42.dll on my C: drive,. Some apparently identical in size, such as in C:\Windows\SysWOW64. Others much smaller, even 1 KB in this obscurely named folder:
C:\Windows\WinSxS\wow64_microsoft-windows-mfc42x_31bf3856ad364e35_10.0.19041.3636_none_72010f573f43ef4b\r
(And another identical, with same timestamp, but ending in \f instead of \r.)
Here's the event 1001:

Terry, UK