The diesel MOT emission test is a visible smoke test. Luckily the engine won't be under any load during the test, so, although an air leak after the MAF can cause smoke due to overfueling you should be OK on a light throttle as there should still be plenty of excess air available.
A bottle of injector cleaner and driving it like you stole it before the test has worked for me come MOT time, and my injectors have now covered over 427k miles.
