Pumps fitted correctly so it engages with camshaft, the pipes on the top of the sender were a bit rusty so I used petro patch to make sure no air was getting in. Worked fine when fitted new fuel lines then wouldn't start the other day
I'd still say its the sender, mine started playing up when I nearly ran it out of fuel too. The pipework was a little rusty aswel and I did the same as you did patching it up. Do you have anyone near you who has a disco that wouldn't mind letting you borrow there sender just to prove that it is or isn't where the problem is. I'm breaking my disco that I had the problem on so I have a nearly new sender if you do need to buy one.
try running from a bottle of diesel in the engine bay -put filter feed in bottle plus return line back in bottle-then loosen injection pump input banjo and prime lift pump by hand (lever)until diesel shows at banjo-tighten banjo and try and start engine on starter,it should run.This will prove if you have a problem between fuel tank and pipework to filter.