Sudden loss of power

So,sudden loss of any go last weekend and absolutely gutless when driving uphill. Tick over is fine.
I’ve changed fuel filter- no difference
Checked (new fuel line) no dif
Turbo& hoses all new(ish) &good as part of my maintenance schedule for a24 year old truck
Pulled out fuel pick up (all good)
Checked (newish)Delphi lift pump , tiny bit of crap in filter
In desperation I’m in the process of ordering new injectors , just got one stubborn one left to pull out. They look ancient but local diesel specialist wants €35 each just to test so might as well buy new. Have I missed something? Surely the ip doesn’t just go wrong ? Also changed head gasket six months ago
I’ve quite happily thrown modest amounts of money at it as I think it deserves some care after 24 years and quite content to keep on top of maintenance but am stumped by this.

Air filter?
Worth a new lift pump too for what they cost, even if you end up with a known working spare

Check the diaphragm in the top of the FIP, make sure it is good and not seized (you can push the top plate down against the spring to confirm). If that is ok then make a note of its position with a felt tip pen and remove the whole plunger to make sure that the fuel pin at the bottom of the opening is free to move. Plenty if info on fuel pins sticking if you do a search

Thanks already checked these things , I’ve taken the injectors out . One was absolutely stuck and I had to take the head off so I could tap it out from underneath. That cylinder was pretty sooty so am banking on a dead injector. Currently waiting on parts.