What can be making my flavel regent gas fire smell when burning?

Tried many things to find out the problem, spoke to the company BFM at Stoke, Had several heating engineers involved, including from the manufacturer. Fire three months old, flavel regent free standing convector gas fire with four burner radiants. Makers said it was, 'newness' burning off! had new micro marble fire place fitted all sealed up and checked, smoke 'bombs' up the flue, all clear. Don't think it burns properly anyway, any ideas?