yep after all that buying bits and dismantaling it fuel pipe to lift pump was blocked with something...
How much diesel is the pump lifting from the tank? Disconnect the outlet side and pump it manually, it should squirt a LOAD of diesel out, no air or anything just pure diesel, if not then suspect an issue in the lines/pickup/fuel sedimenter corroded badly letting in a lot of air, which will reduce the volume of diesel it can pump.
If you get lots of good clean diesel then suspect an issue getting diesel to the injection pump, once you rule that out start thinking pump.