F20 is the code that the machine flashes up when it does not sense water entering. This can be a bad valve, or the water source turned off or even the flood stop hoses that some people use. In this case it was a poorly routed pressure switch hose. The hose was routed in a way that allowed it to rub against the back of the tub. It took 2 years but it finally rubbed a hole through the hose which meant that the pressure was being released and so the pressure switch kept calling for water.
A new hose routed properly and all is up and running again.