UPDATE – just as I was about to contact AO quoting the Consumer Rights Act the fault corrected itself!
I now believe it happened because I had started to switch off the microwave at the mains between uses in an effort to save a bit of power – as encouraged by the government! Now that I have reverted to leaving it on all the time the display has gradually returned to how it should be.
I wonder how many more people will have similar problems if appliances are constantly being switched off and on again