Thanks.
I believe that the rising pressure is normal. I had initially mistakenly thought that the duty cycle went to zero, but it drops to 25% so the pump is still pumping. When coasting, the injectors are closed to save fuel so the pressure rises constantly until you start using fuel again. If the pressure gets high enough then the by-pass valve opens and the fuel is returned to the tank.
I'm going to check the electrical connections on the fuel pump but what a faff to get at it! I've had to remove most of the trim from the boot and all the rear seats.
It's a pity it's so difficult to remove the rear seats, it seriously increases load space!
I'm also going to wire an LED in parallel to the pump to see if it loses power when the issue occurs (faulty module) or whether it still receives power in which case it's definitely the pump failing.