I had a similar problem when I refitted my towel rail. It worked initally, I bled everything and it still worked but the pressure dropped. I increased the pressure and then the burner wouldn't fire up and I kept getting the flame failure light.
After much messing it would appear I had put too much pressure in which is why it wouldn't fire. The instruction manual reckons between 0.5-1.0 bar of pressure but I've found anything over 1.5 seems to trip it out?
