Worcester Bosch Combi 350 Boiler keeps cutting out?

Hi there,

We have the above named boiler and have had 2 WB technicians in this week to look at it and neither of them have managed to fix the problem; In total they have replaced the air pressure valve, 3 circuit boards and the thermistor (I think).

Basically the problem is as follows: When you turn on the heating, it works fine for a while but the flames in the boiler go out after about 12 minutes (I've measured this a number of times and it seems to always be between 10 - 15 mins). After the flames go out, you can still hear the fan running in the boiler. One of the technicians said that that was usual and it would reignite after a little bit. It never reignites. This happens even when I bypass the only room thermostat in the whole flat (i.e. wire it so that the heating should be constantly on).

The same happens with hot water - if you are in the shower for more than 12 mins, the water goes cold when the boiler flames go out.

Additional problem (not sure if it's related or not) is that sometimes when there is hot water demand, the boiler does not ignite: You can hear the spark in the boiler, but there's no 'whomph' as the gas cathes fire. The only way to get it to light is by turning the water on / off / on until you hear it catch (a bit like a dodgy sparker on a gas cooker).

Anybody heard of a similar problem, or got any ideas as to what might be the issue? And any resolutions!!

Many thanks for any help.

Ben.