My MIL has A 53 plate golf TDI, 80,000 miles which just failed the MOT because of the exhaust emissions being too high. Is the car knackered or is this a simple fix?
As already said, it could be lots of things, so a trip to a garage for an estimate should clear things up.
It could be your catalytic converter, as it was in this case:
Before the MOT appointment take the car up and down a motorway for about 15 miles and hammer it through the gears. Do this until no black smoke appears behind you. This will clean out the injectors.
This will cure the high emissions but you must really accelerate fiercely.