Yep, I agree about the 'above knock threshold' code, but as you say, it doesn't always flag up that code.

P0251 basically means that the fuel pressure actual is different than fuel pressure desired.

Easiest way to test the pump and IMV is to disconnect the injectors multi-plugs and IMV, add leak off test kit and datalog fuel pressure during a 5 sec crank. Their spec is greater than 1050bar, a good pump/IMV will produce ~1600 bar. If it does, the pump is fine.

By adding the leak off test kit during this test, you are checking if there is excess leak off from any injectors when the pump is supplying the rail with high pressure. If there is fuel present in the pipes during the 5 sec crank it's not looking good for that injector. (I'm wondering if the garage checking Zemon's car did this test?)

I have been working on a Focus TDCI today with Delphi system, it has severe lack of power/smoking/knocking on accel. No DTCs to give me a clue, road tested & datalogged FRP vs FRP_DSD and difference is up to 80-90bar difference on accel when it's faulting. So carried out above tests and have found 3 injectors out of spec.

During 5 sec crank the injectors in cyl 3+4 had 15cm on fuel in them, vs about 5cm in cyl 1 and 9cm in cyl 2.

Results from 1 min and 4 min leak off test at idle confirmed that the injectors in cyl 2+3+4 are leaking off excess fuel, thus losing fuel pressure on accel.