potterton netaheat 16-22 not turning off?

Have a potteron netaheat 16-22 that does not turn off with adjustments of the programmer or thermostat. Even if the programmer is in the off position and thermostat is set to minimum. Only way to stop it is to turn off using the boiler temp control or turn off the power. Prior to it staying on permanently it sometimes didn't come on at all. While it's running it lights on and off as normal. Programmer relays seem to be working fine. What needs replacing?