Chaffoteaux Britony combi?

Hi, I can't work this out. The boiler has problem running in DHW and CH modes. It tries to start several times then often manages to get going. The burners run for 30sec to 15mins. More often say 2mins. Then stops and tries again.

Various boiler men have played with it but they don't have a clue. The last one said it was the controller card, but a new one of these didn't fix it. A lot of shaking of heads and suggestions that I get a new boiler. But this one is only 6-8 yrs old not new but hardly a museum exhibit!

I took a look and popped a meter across the leads from the air pressure switch. This appears to change as the boiler stops, although I think that the fan is still running. This looks to me as if this means that the air pressure is the cause of the problem. This also makes sense as no red light is shown when the boiler stops whereas if it should if overheating is the problem. It simply tries again.

Can anyone shed light on this ?

Thanks P