Problem with worcester combi boiler pilot light?

Got up this morning and the pilot light on my boiler had gone out, I relit it but although it lights then fires up after about 20 seconds a large click is heard and it all goes out again, it will relight but same again. It sounds as if some kind of trip switch is being activated. This boiler is the bane of my life with the worchester engineers charging �170 just to come out...anyone any ideas on whats wrong with it?