Well, it looks like (he said, desperately looking around for a piece of wood) my infotainment screen may be fixed, without having to have a replacement fitted.
As I mentioned before, when I contacted the dealer about it they first had to contact MG for permission to examine the problem

rolleyes

, then they videoed the problem of the screen flicking back and forth between the home and settings screens.
Whilst waiting for MG's response, during a trip to South Wales, the infotainment screen stopped flicking and displayed the home screen properly. Great, I thought, it's fixed itself. But...no. Unfortunately, the settings screen was now totally inaccessible and neither the radio or Android Auto would work at all.
Then, a few days ago I got the call from the dealer to bring the car in to have the infotainment system reset. I told them I had already reset it etc etc countless times to no avail, but the dealer said that they had been given another reset procedure that had to be followed in a certain sequence.
It sounded a bit odd to me, but hey ho, I took it in anyway. Just 10 minutes later (and no error messages via the app, strangely), the dealer handed the car back to me with a working infotainment system - yaay, I wonder how long that will last I thought.
Well, it's been several days now, and apart from having to re-log onto the car again via the QR code and to redo all my preferences and settings again, all seems well. Well that is apart from the inability to connect using bluetooth and the frequent drop out of the DAB radio, i.e. situation normal for an MG4!
In celebration, I washed the car today...and found my first stone chip. Bitter-sweet eh??