Put some injector cleaner in the fuel, fill up with Superdiesel (not ordinary diesel), then take the car for a good 100 mile run, using 4000rpm occasionally. That will give you your best chance of getting through the MoT smoke opacity test. If it passes on that, yet fails for some other reason, make a judgement on whether to spend the money or not. The expensive quirk of these (Isuzu) engines, shared with the Vauxhall Cavalier, is that the alternator and brake vacuum pump are combined and if that fails it's very expensive to replace. It's also a timing belt engine and there is the possibility that if it has not been changed it could snap during the diesel emission test that requires the engine to be revved to the governor. Yes, you're right that any scrappage discount your could get for the car is unlikely to be more then the discount you could get anyway.