How can I fix my Ford Fiesta's starting problem?

My 1998 ford fiesta 1.8 diesel struggles to start.
All the glow plugs are now working, after changing 2.
first thing in the morning I heat the plugs up 3 times and then try to start the car, this can take up to 30-40 seconds, and then sometimes won't keep running. After a few goes it starts and continues to run and drive perfectly. If the car is re started within a few minutes of being switched off it re-starts fine, but if say 30-40 minutes has passed, although the engine is still warm it still strugles to start. the car has been regularly serviced, and has only developed the fault about 6 months ago, at which time I changed the fuel filter as I was told that this would be the problem, but it didn't solve anything. I have since been told that it could be a seal in the lift pump... Could this be the case, and if so can the seal in question be changed, or must the pump as a whole be replaced, if so HOW DO I FIX IT?
Cheers, Richard..