You don't say what make, model or year the car is but it's rather unusual for an inlet valve to break as such. The main reason would be that the timing belt has broken which can then result in the pistons hitting the valves. Have you checked with the garage as the reason for the valve to be broken? If we assume that the car has a cambelt and has not been changed for a considerable time then this may be the cause. As for the new injectors and given that the engine had run okay for two months we would not at this stage consider that they were the cause of the problem.