+1 The fact that they couldnt find the fault until the 3month warranty had expired is theyre fault.
Ask the manager there for the warranty departments number & phone them
i had a similar issue with my window motor, I had reported it before my warranty had expired but by the time i got the car in for them to look at & they diagnosed the fault it was out of warranty. Sales wanted nothing to do with it & neither did the service dept. Thats why I went direct to the warranty department & they agreed to pay for it