Suspect fuel filter?????

My '03 TDCI Focus fails to start, got harder to start over a 3 month period and was easier when the car was warm. Had it serviced and cam belt changed but no improvement, then took about 15 mins to start and managed about a mile before completely died. No joy restarting on jump leads. Glow plugs seem good when checked. Also had fluctuating RPM between 600 & 900 when idling just prior to it cutting out.

Anyone have any ideas????

Thought it might be the fuel filter as had been running the tank pretty low for a long time and the fault seemed to be progressive.

Thanks alot,