On the Windows 10 machine, a 24" LG, although using a VGA connection as this will have to do until the next general upgrade.

i read somewhere that a similar problem existed with an external TV screen, i'm not really an expert but the problem could be around there. Of course updating drivers is usually a good start...

Since posting my original message I've had the time to check the monitor, which is an LG Flatron E2350 and I've been on the LG site looking for driver updates. Unfortunately, they've produced nothing new since Windows 7. The other LG monitor I use on the older machine is okay with Win 10, but I don't want to swap them over. The annoying thing is that my new Dell machine running Win 10 and using the old screen was fine right up to the update. Now, this problem is so annoying, I think I'll probably need to buy a new HDMI monitor that works with Win 10.

You need to reinstall the driver for video card, not for monitor, usually after big winodws updates.

With Windows 10 nothing is sure.
After each upgrade it is necessary to check the parameters.
The risks are greater for the audio parameters.

And after an upgrade, if nothing is wrong, that does not mean you wont
have to check the parameters again after a new startup.

The rough part is, those win10 updates are involuntary, and they wreak havoc on your proprietary drivers.

https://www.makeuseof.com/tag/take-b...es-windows-10/

You are absolutely right about this. It seems that Microsoft in their infinite wisdom did some driver upgrading and this was the problem. I had some excellent help from Dell's Customer Support and things are now back to way they should be with no more blackouts - hopefully!

Well, I am glad it is ok now, you can block driver update if you want.

Taken care of that already. According to Dell, Microsoft are currently working on fixing this problem which seems to have caused quite a bit of trouble.