Graham, OFTEC engineer in Suffolk here....if it's balanced flue then check the flue terminal. Boulters made a bottom exit flue which sucked back in the gases and upset the combustion process. If your engineer is checking the smoke number with a pump and the CO2 (not CO that's a different issue) with an analysler then he should be able to determine what is going on. Look for 0 smoke, 11.5% CO2 and between 40 - 90 ppm CO pump pressure about 110 and air setting to suit. If it's conventional flue (i.e. chimney) then check it's not blocked! and that the thing still gives the same readings when all the covers are on. Also make sure it can breath properly. i.e NO tumble dryers or extractor fans upsetting/depriving it of it's air supply. Hope this helps.