Firstly, many thanks for the suggestion about the system restore.
I did this and to my disappointment, the terrible flickering was still apparent (I've got severe cataracts and diabetic retinopathy and anything that puts a strain on my eyes is obviously not welcome.)
I should also mention that after the Windows 10 update, the reboot and the flickering, I exchanged the HDM1 monitor on which this was happening for an old VGA monitor that works with my Windows 7 PC. This monitor worked perfectly with no flickering - however, to my disappointment I found that in 20% of cases of turning on the PC, this VGA monitor failed to show the normal desktop but instead appeared to be a screen of fractured glass (that's the only comparison I can make.)
I had to conclude (with my very limited knowledge of computing), there was a fault with both monitors but as it was a totally different fault I wondered if there was a fault with the HDM1 monitor but the one on the VGA was simply because it was old and not compatible with the Windows 10 PC?
Or the Win 10 update triggered some dormant fault in my PC (graphics card?), or the HDM1 monitor had decided not to work properly and it was just coincidence that it was after the Win 10 update?
With the system restore not correcting the problem, I decided to experiment and I reduced the screen size by 30% and was pleased to see that the flickering was suddenly no longer apparent. The screen size I created was far too small to work with, so I kept increasing it back to almost the previous size, and the flickering was still no longer apparent.
Furthermore, I had changed the desktop background (a US nature reserve!) to a very bland one-colour one that Microsoft supplied with the PC. The flickering was still there when it was a full-sized screen but when this was slightly reduced, all the flickering stopped.
I hate 'mysteries' like this but at least for the meantime my PC is usable.
If anybody can shed any light on this I would really appreciate it. What has been an education is reading internet articles about how many people report problems, sometimes serious ones, after installing a Windows 10 update.
In my case I did a system restore back to mid-July and I'm now wondering whether I should no longer do any more updates if this is what happens?
Regards,
David