pour 1l of 99.9% isopropyl alcohol in the tank.... if you got any E85 filling stations add around 30-40% E85 to your fuel...
drive it for a while (50km), then retake MOT (with well warm engine before you hand it to them), ask the tester nicely to drive it round in 1st at 4000 rpm for 1-2 minutes before the actual test.
adding IPA helps to clean injectors etc (much cheaper than injector cleaner products and it's the main ingredients of those things anyway!)
running on higher content of ethanol in fuel lowers emissions... when I used to live in Finland I always ran my car on 30-40% alcohol for one tank before doing 'katsastus' = MOT, same when I went for the emissions test. always passed with flying colours (it had well over 300,000 km on the clock with first cat).