lynall said:

Just had a thought how many holes has the new head gasket got?
Sadly I think you may have to take the motor down.
I assume you have oil pressure etc?
I cant remember which end of the head the engine feeds oil pressure to, but if rocker shaft was adrift that may have some bearing on whats going on.

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The HG that came off was a 3 hole, and the one it's got fitted is also a 3 hole. I checked the piston protrusion and it was within tolerances for a 3 hole gasket, according to the Haynes manual.
It has oil pressure.
I actually managed to locate the problem eggzactly. I moved the injector that was working to a different cylinder and that cylinder was then the one to start working properly. I got the rpm so that i could hear the knocking, and when i cracked off injector number 3 having done the swap, the knocking disappeared. I'm positive the knocking is diesel knock coming from injector 3. I'll order some nozzles and try to recondition my injectors myself instead of paying a fortune for new ones.

You cant get new 200 injectors its recon only, I have tried some with cheapo nozzles, rubbish, you can get decent nozzles, bosch I think?