I'm a massive Fordson fan and really love old vintage tractors and machinery!

I have a 1957 Fordson Major Diesel, it's a great tractor and the farm's workhorse.
However, last week I was discing a field with a twin disc trailed implement that wasn't very deep in the ground and the tractor coughed and spluttered to a stop?
I was discing for about an hour before hand and everything was running fine; I tried to restart it, but it had no smoke and was turning like the stop control was pulled and no diesel was pumping. So I pushed in the cold start button and retried it; this time it fired up for about 2 seconds at a very low RPM and then stopped? Fearing for the worst, I took the cover off the injector pump and observed the rack and plungers, when the starter was operated they all moved up and down correctly and followed the cam. Then I noticed that the glass bowl on the lift pump was only half full of diesel and there was no resistance in it, which was quite odd as the tank had nearly 20 litres of Diesel in it? So I decided to bleed the system, when I took the bleed screw out of the secondary filter on the outlet side there was a lot of Diesel pressure and it sprayed up. I then operated the lift pump; resistance was felt and it's glass bowl filled up with diesel, it also started running out the bleed screw, so I re-tightened the screw and then opened the bleed screw on the injector pump. At this point, when I operated the lift pump bubbles started coming out of the bleed screw and then flowing Diesel. So I tried to start it, but again it only ever attempted to start with the cold start button pushed. So I disconnected the outlet pipe on the lift pump to see if there was a blockage, but when the priming handle is operated it spurted out a bit of Diesel. I then disconnected the inlet pipe into the lift pump and Diesel was flowing out of the tank easily? I'm thinking the lift pump is faulty as resistance is only felt when the priming handle is close to the lift pump body? Any ideas?
I really hope that it isn't anything major, but I don't think it would be as it was running very well before.