A problem with my Glow Worm 70FF Boiler?

I have a Glow Worm 70FF boiler with an intermittant fault. After a 'timed off' period the boiler sometimes does not start up at the beginning of a 'timed on' period. When I check I see the Reset light on. If I turn the temperature control knob fully anticlockwise and then clockwise back to the normal set position the boiler lights up. It then continues to light up all through the 'timed on' period. So the problem only occurs after an extended shutdown period, perhaps once in every 3 or 4 starts. Any ideas as to what the problem is?