Myboiler when in central heating mode will heat the radiators until the set temp is reached then trip out as it should do.
when the thermostat calls for more heat the boiler will sometimes relight but 99% of the time will not light,turning the on off switch to off for a short period of time then back on again cures the problem until the radiators reach the set temp again then the process repeats.
The boiler works perfectly in hot water mode boiler lights every time the hot tap is opened

Neil Johnson, October 2005

I have the same problem, I posted the same question today. After looking around the web other people also suffered similar effects and all pointed to the ignition control box pcb. So I removed it to have a look. It is mounted inside the blue box held on by a single screw. You need to disconnect all the wires (after isolating the power of course)Then it will unplug. A new one costs about £70 plus vat, however upon close inspection I noticed that one of the curled tags on the pcb which make contact with the four connectors of the boiler was bent meaning that only one side of that particular conection was making contact and then only just. I had to remove the pcb from the blue housing which is held in by two small raised edges just where the four boiler connections come through and then carefully reposition the bent tag. This could explain why the problem was intermittent as heat and dust could all have an impact on the poor connection. Also whenever I previously looked at the boiler and fiddled about it seemed to work fine for a while as I would usually give the blue box a wiggle which must have improved the continuity a little.Anyway it seems to be working better than ever now, if the problem returns I will simply replace it as I am now sure the problem lies here.

Christopher James, November 2005