It does not happen every time but is fairly frequent that the central heating will stay on when the timer says it should have gone off, when this happens it stays on constantly even if it gets above the temperature of the thermostat. It usually takes a combination of trial and error swithing the heating on manually and clicking the temperature on the thermostat up and down to get it to turn off. This morning however this does not appear to be working so at the moment we have turned the whole system off. Is this likely to be the timer unit that needs replacing?