Diesel injector woes.

Have a berlingo with a 1.9 diesel engine, only 30k on the clock.

It broke down the other day but was repaired under warranty-one of the injectors was "Stuck up" (Garages term, not mine). I assume this to mean that the internal mechanism was stuck in the 'up' position, rather than it was clogged?)

Since then it has broken down again..same problem but a different injector!!

What are the possible causes of this?

Current thoughts are:

1. a batch of faulty injectors were fitted when the car was new.

-or-

1. Some sort of problem with the fuel. (Contaminated?)

Is it possible that a problem with the injector pump could cause the injectors to stick open? Too much pressure maybe?

I may decide to replace the remaining injectors, the logic being that they have done the same mileage and are from the same batch so (In theory) are in the same condition.

Never having bought injectors before, do they need to be 'calibrated' before fitting or can I simply fit them as supplied?

Any gotchas I should be aware of when fitting injectors?

Is it worth using a bottle of injector cleaner on a regular basis? If so, what's a good (And easily available) brand?