Intermittent fault with combi c/heating

[saabman]

Hi. I have an intermittent fault with the Worcester 240 combi.I have no problem with the hot water side of things but the c/heating is very temperamental.
When the timer kicks in the boiler will not always fire up.The pump will run and continue until the end of the timed sequence or is over-ridden manually.On other occasions the boiler will fire up and the cut out after a few minutes.It may fire up again,but not always.I can sometimes hear a switch on the circuit board rapidly opening and closing as it try's to fire up.If I turn on the kitchen hot water tap and allow it to run for about five minutes the heating system may stay on,but again not always.I have no thermostat control system in the house or on the radiator valves.The timer clock is new.The radiators are flushed and bled.
Any ideas...Many thanks