Check the fuel pipes at the fuel tank. Remove the boot carpets, undo the 6 screws holding the tank cover on and see what the Nylon to pipe fittings look like. If they have cracks in them then they're likely to be the culprit, letting air in. Only real cure is replacement sender unit, even epoxy doesn't work too well or for long ...
I know this has been done a million times, but what brand of lift pump did you get? Was your car working perfectly OK before the lift pump was changed?
These are self bleeding systems so if the pump is working OK then it should be a case of fit, crank it over and away you go.
Here at LandyZone we have plenty of very knowledgable members so if you have any questions about your Land Rover or just want to connect with other Landy owners, you're in the right place.