Emissions atm...my thoughts why?...most of us arnt out and about doing the miles we 'used' to and subsequently the system doesnt get hot enough so the cat/o2 sensors dont read as they would under more normal circumstances,
Use a bottle of catclean. Follow the instructions and before the MOT drive it like you stole it on a safe road so it gets nice and hot/clean before its tested.
Air filter/plugs etc check and replace if neccessary. It wouldnt hurt none to spray carb/brake cleaner down into the throttle body either. Make sure its clean and seats cleanly in the bore. I use a toothbrush.