If you believe it's crap in the tank then start by removing the pipe off before the lift pump, drop the pipe as low as possible and see if Diesel flows out! If not blow up the pipe, if you cant blow then there is a restriction (Drain and clean tank) Blowing may clear blockage does it flow now? if not try sucking, if no joy check pipework back to tank, if no joy drain and clean tank. If it did flow without without blowing / sucking reconnect the pipe and disconnect after the lift pump, pump the lever on the lift pump, no fuel comes through then crank it over, if no fuel replace lift pump. If fuel came through reconnect pipe and move up to the fuel filter, if it has a bleed screw undo it or take the pipe off after the filter, crank it over, any fuel? No the filter is blocked, replace filter. if fuel flows through then re-check solenoid on injector pump, turn ignition on and remove and replace wire, does the solenoid click? (assuming the wire is LIVE!) if not sticking solenoid, remove, does the plunger move freely? if not clean to free it or replace it. If there are no issues and there is fuel to the injectors but poor as you mention, then try a "small" squirt of easy start in the inlet manifold. Personally I would start at the filter, you could fill a bottle with diesel and connect after the filter, if it runs then work backwards to eliminate filter / lift pump. Not knowing the vehicles history you should go with your gut feeling, the above should assist you to diagnose the issue, it's up to you at what point you start at.