In the end I scraped all the paint on the bottom out because it was just flaking off everywhere and now have a lovely rusty bare metal bottom to the microwave which I suspect would seriously freak out any female who tried to heat anything in it but fortunately there are none of those chez moi and I don't really give a stuff if it looks like shit inside as long as it cooks stuff. I also ditched the plastic thingy so the whole weight of the platter and food rests on the central axle that turns it. That can't be doing the motor underneath much good and it grinds and squeaks a bit but still carries on working. It's also already seriously rusty everywhere else inside, not just on the bottom where the paint got worn through.
I'm a twat and should have known better because the previous microwave was also from Tesco, had the same design flaws and went exactly the same way. In fact within 2 years that one had rusted right through the bottom panel in several places so you could see the counter top underneath!
So firstly to anyone thinking of buying a microwave from Tesco - DON'T. They are unmitigated crap. The Ford Fiesta XR2i of the microwave oven world. You might buy cheap but you'll pay dearly in the long run when you have to replace it in a couple of years.
Now the first microwave of the three I've owned which I bought back in the 1990s lasted 20 years and looked as good when it died as when it was new. I can't remember which make it was. Something electrical inside it broke and wasn't worth fixing. So clearly they can be designed to last a decent length of time.
So is it just a matter of avoiding the awful awful awful shite that Tesco sells and buying anything else at all or do more reputable brands also suffer from these sorts of problem?