As an update on the above, I brought the car to my MG garage and, after two visits, this issue is now resolved - but not by the garage I hasten to add. They advised me that it might be because one of the spark plugs is gone or that, because the heating system needed the ICE to fire up, it was not allowing EV mode.
After some experimentation, it seems that all the elements of the heating system (air, seats, windows) demand power from the traction battery and, if the demand is high enough, then the ICE fires up to protect the traction battery, regardless of the remaining battery charge level. This continues for some time during which EV mode is not supported and you get the message I was getting above. However, after some time running in ICE mode, it seems that the batter recovers sufficiently to allow you to go into EV mode - always if you reduce the heating demand (turn down the temp sufficiently, for example) or sometimes even if you don't reduce anything. I assume the latter case is where one or more of the heating elements happens to be off on a thermostatic cycle at the time you switch to EV mode. Finally, this thermostatic arrangement probably explains why the car drops our of EV mode sometimes of its own volition, without me changing anything.
This is my new understanding and may or may not be accurate but, either way, it perfectly explains the behaviour of the car is all circumstances with respect to the ICE/EV mode set-up and I am now using it to maximise my EV driving hours without going insane trying to figure what is going on with the car. A win-win, I feel.
