If you could, thanks
the simple answer is no the different injectors are not interchangeable.
The cylinder heads are different to suit the 2 different injectors, with the earlier 29.805mm tip injector the cylinder head is manufactured for the end of the injector nozzle to sit in the correct position in the combustion chamber, this injector also has 6 holes where as the later 22.155mm tip injector has 7 holes to improve atomisation of the fuel for emission purposes. Obviously the approximate 7mm difference from the sealing washer to the tip of the injector between the 2 injectors would make a huge difference to the injector tip position in the combustion chamber.
One quick point is with any fuel pressure concern on this engine if the fuel filter hasn't been replaced recently then before replacing expensive parts replace this especially if the codes were P0251 and P1211.
I hope this is the information you require but if I can assist further let me know and I'll reply as soon as I'm able to.
It is as I expected, then, and why I never tried fitting them.
The fuel filter was one of the first things changed.
If I recall, the P0251 error code did come up, but I don't have access to my notes or the car right now. I suspected the injectors as two of them are noisy (you can hear a "squeak" as they are actuated, if that makes sense, and not just the normal ticking). Unfortunately I've not been able to do a leak test on them. There is also a light amount of white diesel smoke from the exhaust.
It is a high mileage car at 197k miles, so knackered injectors are a definite possibility, along with the diesel pump, etc.