Diagnosing 4x injectors was understandable (depending on the fault code that is!) and the price seems fair.

it's common for one to go and the rest to follow.

However, there are a multitude of things that can go wrong with the car, and soooooo many garages automatically claim that it's the injectors (because 9 times out of 10 it is) but fail to test them properly. £15 per injector is too cheap for them to be tested by a proper Delphi specialist - and Ford are definitely not Delphi specialists. If the specialist wasn't a Delphi one, then they can not test them properly. They can only test SOME of the parameters, not all.

You need to know the exact fault codes.

I've unfortunately had a lot of experience with misbehaving TDCis and if you can find out what codes the car flagged up I should be able to help more. Feel free to PM me.