Glow Worm Ultimate control unit fails to switch off pump?

I have a Glow Worm Ultimate Boiler which has a control unit with the following code ULT 100F(its about 10 years old).

The problem is that when the system is off (e.g. no hot water or heating required) the pump continues to circulate water in the system and will continue to do this until the knob on the control unit is turned fully off and then on again. Does anyone know why this is happening and what the cure is?

Details - The boiler just about keeps pace with our heating needs, but has recently stopped switching off the pump. The pump will run all night unless the control knob is turned to off and back on again. The control unit knob is normally set to 3/4 on. This can be set to zero if the boiler is left off for long periods. There appear to be three water circuits: (1) radiators, which is controlled by its own valve. (2) hot water, which is also controlled by its own valve. (3) a redundant and permanently open circuit which appears to relieve the pump flow when the other valves are closed.