Starting Problem

Morning everyone
this is my first time on a forum I need some experienced help, my discovery 2004 TD5 has developed a starting problem, it turns over perfectly but will not pick up. if I keep the starter motor engaged for a few seconds it will pick up and run perfectly but it is slowly worsening over time.
I feel its a fuel problem like fule is siphoning back to the tank thus the fuel pump has to re-pressurize the fule lie each time I turn the key.
I have changed the valves in the fuel filter housing first the air bleed valve it seemed to improve for a couple of days. I then changed the non-return valves in the filter housing, the reed valve in one of the connections showed defects so I was sure that was the problem solved WRONG the problem still exists to a lesser extent. I am now suspecting the fuel pressure regulator could be allowing fule to return to the tank via the fule cooling line, is this possible , Does anyone have any ideas ???

Have you tried doing the self prime (check the manual) before starting?

How good is your battery, I had similar issues with mine. Car would start no problem but low voltages on ecu's threw up TPS fault.

hi, all thanks for the response,
Have found the problem, I think !!!, the fuel filter housing had a leak at the copper washer seal due to corrosion at the tailpipe connection sealing face furthest away toward front thus there was air ingress and siphoning back to the tank .the leak became visible after I renewed the duck-billed non-return valves in the filter housing instead of air ingress the faulty seal created a diesel leak. New housing has been ordered, hope problem solved !!! I realize I may have created the leak myself by disturbing the seal to replace the valves but the housing is corroded so replacement is not a waste. once new housing is fitted I will have an answer one way or other
Thanks again