Glow worm betacom 24c, hot water not working?

My betacom 24, is working perfectly on heating, but when I open a hot tap, the burners fully light for about 10 seconds before cutting out. Neither the hot outlet pipe nor the heating flow pipe from the boiler get hot whilst the burners are on so I don't think it's anything to do with the diverter valve? Any ideas would be greatly appreciated, thanks