Three faulty injectors?

In the past 18 months my 2003 TD4 has gone through three fuel injectors, and I have just noticed that it is once again displaying those all-too familiar symptoms of difficult starting, juddering at tickover, white smoke etc which leads me to believe that injector number 4 is also on its way out.

This all seems to me to be extremely unlikely (the car has done just over 95000 miles). Can anyone shed any light on what is causing these failures, or perhaps, might it not be the injectors at all? If this is the case I will be pretty annoyed because I have forked out a small fortune so far on the assurances of my mechanic that this was definitely the problem. But if it is not an injector problem, then what is it?

Any thougts or ideas which people might have would be most welcome.

Given the mileage perhaps it was just a case of all four injectors needing a refurb at the same time.

How much does it cost to have injectors refurbished?

The firm we use charge £69 + vat for a full refurb. At 95k then yes it would not be unusual to have had to have all 4 done. We have had to do injectors at any thing between 80k and 130k

Take it that's per injector?