P1211 and 0251 are two of the most common codes on the TDCI. If anything it's more likely to be pump (or inlet metering valve) related rather than injector. When injectors die they usually (but not always) bring up individual injector fault codes.

The worst thing you can do now is just replace parts at random. You could very well find you swap the injectors (expect around £200 for fitting and recoding) and it's exactly the same.

By far your best bet is to take it to a Diesel specialist (preferably a Delphi dealer) and get them to diagnose it. They can check the injectors and pump to see what is at fault. If you don't get it properly diagnosed you will end up replacing parts at random, it will cost you a fortune and you will regret it.

Cutting corners now will cost you in the long run. Been there, done that, and have seen a LOT of other people do the same.