Replace one or all diesel injector(s)?

My 2003 Mondeo TDCi "dropped the code" on one of its injectors around Christmas, and although my mechanic (a diesel specialist but not a Ford dealer) re-programmed it so that it drove well again, he did not replace it. The problem has now re-occurred, but he now recommends going to Ford to have it replaced. Is it OK to replace just one faulty injector, or should all 4 be done together? Doing all 4 would obviously be a better option, but also 4 times the cost, so not a cheap job! Thanks

They are sold individually, see here, [broken link removed] so I assume they can be fitted and coded individually.

I replaced 2 last year, cost €600. Would wait for them to go before I would replace. Slim

Have been advised by two independent diesel specialists that once the car is over a certain mileage, say 100k, it is advisable to replace all at the same time. However, this may depend on the quote! Maybe get one done and then time for a change of car!