call baxi/potterton, they are the makers and will fix it for you. BG are a load of salesmen on commission who can't be trusted!
the powermax is a badly designed boiler, why put the flue outlet on the back and make it only a push fit. it's prone to corrode and is probably impossible to replace without removing the entire boiler, that would be if the part was even available. in the diagram it looks like a waste pipe on the back right of the boiler.