i take it it's a promax combi? if so your condensate pipe is most likely frozen, the white one coming out of the bottom left of the boiler, this causes ignition lockout. best to get it seen to quickly as they are known for flooding the gas valve and also damaging the fan, very pricey. if it's been happening since before the cold weather then the combustion ratio's probaly need adjusted, again a job for an engineer.
Ian
January 2010