Fuel emission failure and injectors

Have a rover 214 now failed mot 9 times on emissions despite changing lambda sensor 4 times ( very costly) entire exhaust system and catalitic converter. Now the garage are taking it apart again to now look at the fuel injectors but CANT GUARANTEE this will fix the problem. This is gonna cost another 500 pounds at least! Can someone tell me whether after changing everything that has been changed that this will work? I am assuming they are going to clean the injectors? The engine blew up and I had a new sump put in and the head skimmed with a new upgraded gasket kit put in. Since then it has had all these mot failures and problems with the gasses that are more than 3 times what they should be even with all the new parts. IS THERE ANYTHING ELSE THAT IT COULD POSSIBLY BE? Am I stupid in thinking that this is a car. Something mechanical. And everything can be fixed? Tell me that this can be fixed! And if there is anything else that can be looked at while the engine is yet again being dismantled! Help