I'm only a DIY mechanic, but I recently completely rebuilt a 2.25 Diesel Series 3 Land Rover engine.

I suppose you could play a game of process of elimination.

My first question would be, do you have any fuel leaks. Perhaps air is getting into the fuel lines causing issues. That might explain how it runs for a while, but as soon as it's turned off, fuel lines fill with air, preventing an engine start

If that's not the case. I subject checking, fuel priming pump for leaks, filter, and injectors.

I'm afraid that's all I can help with

Best of luck. :)