I would be interested to know how these injectors are failing. Are they failing because of a problem with the electrical circuit, or because the nozzles are becoming worn? For your garage to just say they are 'faulty' doesn't tell us much, or help them, or you, to understand the cause. As said above, a good diesel specialist who has all the correct equipment for testing the system pressure and the components would be far better placed to help you. In all fairness most independent garages don't know half as much as they'd like you to think when it comes to areas of speciality such as this.

A technician in a garage near me removed the injectors on a Focus to replace the seals. Conscientiously he thought it a good idea to clean them prior to re-fitting, so he did so on the wire wheel. What he had no understanding of is that although there would be no problem cleaning the body of the injectors on the wire wheel, he also wire brushed the nozzles, leaving four previously perfectly good injectors totally destroyed. He didn't even know what the problem was until he discovered he could not start the engine, and even his boss didn't know until someone from outside the business told him.

Moral of the story. You cannot trust people who have limited understanding of modern cars.