My combi boiler ignites flame to heat radiators then goes off itself?

When my timer clicks on, my combi boiler clicks and ignites the flame then the bigger flames comes on to heat the radiators, but after just a few seconds it switches itself off again and all flames go out. It tries to restart it again, but the same thing happens. I tried running the hot water tap and the water went hot fine with the flames coming on, then the heating stayed on. Very strange. What can be wrong? and what needs replaced. I don't have cover due to it being about 15 yrs old, but has been a really good system.