So it turns out it was not a hardware issue or a driver issue with AMD's graphics card. In fact it seems it was related to the OS. Apple this morning released a new version of Catalina (10.15.7) with the following line in the release notes:
"Addresses a graphic issue that may occur on iMac (Retina 5K, 27-inch, 2020) with Radeon Pro 5700XT"
It did then indeed affect all of the iMacs with that graphics card and those who weren't seeing it just hadn't come across it yet, hadn't noticed it or perhaps had connected an external monitor (my workaround) where it didn't occur.
Here are my thoughts: