My Glow-worm Energysaver 50E has started to make large bangs and then re-ignite itself. After a few cycles of the loud bangs it blows itself out and cuts off. When I go to the boiler it still seems on but no flame is present. The only way to get it started again is to turn it to zero and then back onto one. The whole cycle starts again. I have had a corgi engineer to look at it and he changed the fan as he believed that could be causing the problem. When this didn't work he serviced the boiler after a drain down, but this hasn't stopped the problem of the bangs. These are very loud. Please help. A friend has mentioned having a new burner?