My Potterton 80e often doesn't start up anyone know why?

My Potterton 80e gas central heating and hot water boiler often won't fire up on its own .

We tend to have the Honeywell timer control set to Continuous in winter, and run the hall thermostat at 20degress, and turn it down when we go to bed to about 16 degrees; until a couple of weeks ago, the boiler would turn itself off and on, as it should, but recently it stopped firing up during the night, with the reseult that the house was cold when we got up at around 7 a.m. The hot water timer is set to come on at 0630, and this also isn't working.

The boiler starts when I press the reset/manual startup (?) button underneath it, and generally works and gets the heating up to temperature, but once it's got there (and it doesn't get there every time), it often won't start up again on its own, when the temperature drops below the 20 degrees (or 16 at night). Hot water is fine too when I do a manual start up.

Initially I thought it might be an overheat problem, we found some coolspots on some radiators and have flushed and balanced the rads, so that when the boiler is working, the rads are ok. But I don't think it can be an overheating problem as the boiler fails to start at night or during the day when the room temperature drops below the temp set on the hall thermostat, and its clear that the boiler hasn't run for hours overnight - probably since turning the hall stat down from 20 to 16.

Anyone got any ideas please?

Many thanks in advance - Neil