Injector Problem

[clueless chancer]

My increasingly unreliable mk3 tdci 130 (6) now has another problem;

The enginemanagement light is on, had it plugged into diagnostics, and the fault is low fuel rail pressure. After running the full test, it showed that injector 3 was the problem.

The car is doing less than 40mpg now, even on the motorway, and on start up the idle is very rough, with lots of grey ish smoke.

My question is this: can I just have injector 3 changed and re-coded, or do I need to do all 4?

Also, any indication of cost would be appreciated, if any of you have had one replaced recently!