Many thanks indeed to all who have taken time to write. Naturally given the nature of the manifestation of whatever the cause of the problem is , all views taken very seriously.
Firstly apologies, the full information about the boiler is that it's a Glow-Worm Energysaver 50. GC 41 319 70...
We have a problem Houston, minor explosions in the boiler! Particularly prevalent when on high flame. Perhaps a clue to the problem is that when listening closely with the boiler lit it gives off at about second intervals a kind of pah pah sound which is only really noticeable on high flame...