well they should have left you a report on both occasions with a list of parts replaced, so what did they do? I would get them back if its less than 12months since they came.
the first time they came im not sure what they did but the boiler worked fine for a while and then developed a fault on the hot water side so i got them back out and was told it was a different fault from the first one so they said i would have to be charged again. On both occasions i was not left with a report into what they had done. After the fault with the hot water had been fixed a couple of weeks later the boiler the developed the original fault again. I understand that i may have grounds to go back to them on the understanding that the original fault has re-appeared but im a bit dubious to do this as they insist that you give them your card details so that they may charge you if its a different fault.