Wickes Combi 2000 Halstead burner not igniting

[eddy13]

Hi, I have a Wickes Combi 2000 by halstead, which has this intermittent problem that I can't seem to get fixed. Currently the burner isn't firing for either CH or DHW. pump starts and fan does too, and the ignition sounds like it wants to click in but nothing actually fires. The pressure dial also drops to below 1bar.
It did it today, I turned the boiler off at the mains, 3hrs later, i turned it on again and it worked for 10 minutes, stopped once more and now won't ignite again.
This keeps happening, especially over periods of extended use ie when we have heating on all weekend. Each time we get a plumber out, but they can't find the fault, and then it suddenly starts working again for a few months without problems.
I thought it might be because its overheating, but the overheat lockout light has never come on. I also think the pump appears to run constantly when CH is on, and burner is off.
Any ideas what might be the problem?

 

[twgas]

First things first, get the pressure up to about 1.2bar when system is cold.
Then try turning it on (hot water first) and see what happens. Keep looking and listening and make notes of what does what and when. Once you've got as much info on the hot water as you can, repeat the process with the CH.