I used to have a oldish diesel peugout which failed its MOT on emissions. I brought some stuff -(sorry no idea what it was called!) from Halfords (Fairly small bottle) that you added to the engine for this problem. My dad also told me to take my car for a longish drive before i took it to the garage to warm the engine up before it was tested and hey presto it passed that time. i dont know anything about cars and if this 'technique' is relevant for a petrol engine tho! :-)