unburnt gas in flue gases, glow-worm 100e electronic combi?

Boiler fires ok on DHW and CH, plenty of hot water and no unburnt gas in flue gases on DHW, but in CH mode boiler begins to kettle and keeps on cycling, but does not go to lock out, terrible smell of gas from flue, checked with my gas sniffer and its full of unburnt gas, Going to ring G-w technical, but has anyone else come across this problem?