Boiler not coming on - thermostat, timer? Furnance won't light. Puma boiler?

My potterton puma boiler has broken. The hot water works fine but the heater will not come on unless you move the thermostat above 25 degrees. Usually it is on a timer to come on at 6.30 in the morning but it is no longer firer up to come on. I have checked the timer and this seems fine and the pilot light is on. the heating does come on if you manually move the thermostat dial to high. Does anyone have any ideas what this could be? Many thanks Jess