Vokera Compact 28 cuts out at ?

Hi,

I've had a few problems with my boiler red light (of death!) on me since september, but now seems to trouble keeping alight when pressure is any less than 2.5bar. It can sometimes cut out immediately others it will attempt to light.

Its even sounded as if its had trouble catching alight. Everytime i've called out british gas to look at it the thing decides to work again (for about another 2 weeks)

Any ideas would be appreciated!