My Potterton Prima F80 is not shutting down?

After many years of service my boiler has just developed a strange fault. Even though the time controller (Honeywell ST6400C) is in the 'off' cycle, the boiler is still running. It shuts off periodically when the boiler thermostat tells it to then fires up again. Moving the sliders on the controller makes no difference. The only way I can get the boiler to shut down is to turn off the system.
I have a S-type system with a pair of 2-port motorised valves.