Greenstar CDi Combi Boiler, Burner Indicator Won't Light?

I wonder if anyone can help
Last night my boiler's burner indicator light went off. I noticed that the water pressure was near to zero, so topped it up. The boiler goes on for a short while (second) and then the burner indicator goes back off. I've noticed that when I switch it on or reset, it will go back on and when the numbers on the combi rise to over 55ish it will go back off. I don't know what's going on... Can anyone offer any suggestions?