Lift pump has gone, maybe. Crack open the bleed screw on top of the fuel filter housing. Ah, does the Discovery have the fuel filter in the engine bay? Does on Defenders. Anyway, open the bleed thingy downstream of the lift pump and see if you're getting any lift.

They seem to sit there happily doing not much of anything until you start messing with the fuel system, and then they fail and make your life a misery.

Cheers
Simon

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96 Defender 130 crew cab
88 Isuzu 110
ex 85 Isuzu 3.9 120 ute