Hi, i've got a problem with a Worcester 240 balanced flue combi in which the emissions test was very high when tested (1100ppm). I believe the cause is a corroded inner flue duct which is letting fumes mix back in with fresh air via the air intake. When you take the flue hood off above main heat exchanger, the damaged part is the inner flue metal that starts from the boiler body and is cut about 300mm. (There is then an extension piece which is attached to the terminal which slots over this 300mm piece from boiler back) I need to know if the 300mm inner flue piece is attached to the boiler back plate and if this can be changed or is it a case of a new boiler??