Suppose if you've had a bad experience with MSI, then the Strix B550-F will be one to look into.
To be fair to MSI, I hadn't pursued the matter after my first contact with them.
I've since mainly been trying fixes as found on web searches.
I decided to contact MSI again on 30th May though.
They very quickly responded with
"Thanks for contacting MSI technical support.
Regarding your concern, please unplug 24 pin power cable, take out motherboard battery for more than five minutes and reinstall it. Then please enter BIOS, press F6 to load optimized defaults, press F10 to save and reset.
After that, please enter Windows, use Windows Update to install all the updates, download and install latest AMD Chipset Driver from MSI website to check again.
If the issue remains, please enter BIOS->OC to set Global C'state to disabled to test. Thanks!
https://www.msi.com/Motherboard/support/B550-A-PRO#down-driver&Win10 64
Thanks for your cooperation in advance!
Best Regards,
MSI Technical Support Team"
I'll give it a go in the next couple of days. I've tried most of that previously but not necessarily in that sequence. However this time I'll do it in the order they suggest.
I'll follow their instructions and update this thread with results.
Thanks for your suggestion.