How often is it happening Pete? What's the time span?

Normally when the light goes 'on' the thermostat thinks that the space is colder than you want and the heater heats up. When the light is not on then the space is at or above the temperature you have set and the heater does not operate. The light should be 'on' or 'off' for a sensible period of time, say 15 minutes on a cold night to much longer 'off' on a milder night. It should not keep flicking 'on' and 'off' every few seconds/minutes.

Just what I was thinking Bulbaholic, that's why I was trying to ascertain what Pete meant by going 'on/off.