Simon,

Do this second thing first.

Check both the primary(clear or white) small fuel filter in the line from the tank to the lift pump on the side of the I.P.

Check the secondary filter (spin on , right next to the power steering resvoir)

There is a "sock type" strainer in the fuel tank, but if you have to go there , you
have massive infestation.

Clogged or partially clogged fuel filters will exihit the type of symptoms you
describe.

In the interface between Diesel fuel and H2O (condensated out of the air in
the tank) a bacterial growth can occur,which feeds on the interface.
Full tanks ( no moisture laden air ) and biocides [and fresh fuel] help.