Why does the burner on my Ravenheat RSF 820/20T keep switching off?

When I switch on the hot water the burner ignites with a perfect blue flame but then after 20-30 seconds just as the water is getting hot, the burner goes out. I have also checked on the central heating and just as the central heating pipes are getting warm, the same thing happens. I have also tried leaving the central heating on and after about 1 minute the burner once again ignites for about 20-30 seconds then it will go out again. I have tried adjusting the heating control thermostat and hot tap water control thermostat. This has made no difference the problem is still there. The Ravenheat RSF 820/20T is approx 8 years old. I have taken off the front cover but I am unsure what i should be messing with? I would like to have a go at fixing this problem myself so could anyone point me in the right direction?