Thanks (and sorry for the delay), but I don't think it is the heater. The machine seemed to be heating OK in the part of the cycle it performed.
I gave up on trying to diagnose the fault myself and called in a service engineer — but by that time the machine had begun to work again and to complete its cycle! He said, from my description of the symptoms and of the point in the cycle where it had been stopping, that it seemed to be an inlet valve sticking. If it happened again, he said, I should listen for a faint hum from behind the kickboard, and that would be the valve sticking where the water is trying to re-enter. I could try getting a new inlet valve from the internet.
This is where I come back to the forum. There are several internet sites with many parts for the 71340, but nothing which I can recognise as an "inlet value". Probably it is called something different. Can anyone point me to it?