long crank time poor starting and won't rev, HELP!

just had a call from the bro in law and his disco N reg 300tdi has took a good deal of cranking to start up for a good few months
we finished the gear box change last week after 3 weeks of the car not being run
and now it takes even longer to start and will only fire up if the bleed screw on the top of the fuel filter is open, close the screw and it won't rev or if it does rev it will cut out straight away.
he's fitted new battery, cleaned out the sedimenter, new feul filter fitted only last month, and checked for leaks but has found none,

could this be the lift pump?
or has someone else got other ideas?
thanks
Nick

If your having to open the bleed screw it sounds more like it's pulling air in somewhere behind the filter to me, I would be checking the fuel pipes and connections for leaks.

Agreed. Sounds like the sender fuel pipe connectors. In the boot, remove carpets, undo six screws and remove tank inspection cover, have a look at the top of the sender unti, three pipes into nylon 90 degree connections .. likely they're rusty and split. Otherwise, check fuel pipes all along their length but particularly at junctions and bends for chafing etc.