28cdi?

recently had a leak on roof were flue exits had roofer come round and repair but he had to take flue off ,when all was repaired the flue was replaced,i turned on the heating and the boiler ignites and runs for approx 30 seconds then the flame light goes off then about another 30 seconds later the central heating in dicator flashes approx 1 per second i have looked thisd up and it says ignition lockout i have tried to reset but on starting the heating again it just repeats this any help plz