boiler sometimes doesn't ignite?

We have a Potterton Prima F and occasionally we find it hasn't come on at the programmed time in the morning. When we press the restart switch, it does come on straight away, however. This morning it came on at 6.30 and because it was so mild and we don't have a room thermostat (just radstats which are set to comfort level when we are sitting down) and I had my housework to do, I turned it off at the programmer after breakfast. When I turned it back on this afternoon, no heating so another trip to the garage and the reset button. Can anyone tell me what the problem is likely to be here. Boiler was set to 5, pump on max. Boiler was serviced a couple of weeks ago and the chap who did it says he thinks it must be the pump which needs replacing; expensive quote. I am wondering whether it might be the overrun thermostat or the overheat thermostat. I'm not intending to attempt any repairs myself; just don't want to be ripped off by paying on a trial and error basis. The expansion tank has the right amount of water in it by the way.