Glowworm Energysaver 60 - high flame setting not working

[Boxy]

I have a Glowworm Energysaver 60 boiler - when the room thermostat demands heat and the boiler is set with the 2 position flame setting switch to high and the boiler thermostat set to any position from half to max - the boiler starts for a couple of seconds, then switches off, a couple of seconds later it switches on and then off again (there is a relay inside which I can hear switching and the green "demand" neon lights, then goes off again) continuing on like this until the demand is satisfied (ie the pilot light and burner work OK) - If I set the two position flame switch to low the boiler works perfectly - although obviously not producing as much heat to the radiators.

Can anyone give me an opinion on which part might have gone faulty - whether within the boiler or not

 

[Gas2Air]

Faulty gas valve or high/low rotary switch. You need an engineer in to test all components really. Sorry.

 

[Boxy]

Thanks for your quick response - I will get someone to check it

 

[nor750ton]

Thanks for your quick response - I will get someone to check it

What was the result?
Thanks