disco td5 starting problems

hi every one, i have a disco td5 that has a terrible starting problem, hard to explain all the problems but i will have a go.
when i go to start it from cold you can hear the fuel pump whining but the engine just keeps turning without firing, you can hear it trying to fire but with no effect, if you pump the peddle hard it then starts to fire and runs like a bag of s...! as great large clouds of smoke ( blackish) come from exhaust, sometimes you get better results not pumping the peddle or priming on key first, depending on the mood its in!!
its also a bugger to start when warm doing same thing and sometimes fine, or sometimes your running down road ok engine running well, then you have to brake and it will cut out and be a sod to start again, but as a rule when engine is running it goes like a dream, no power lose, sounds good and fuel consumption ok.
i have changed fuel pump/ sender unit, replaced washers and seal on injectors, had fault code readings checked with no problems, checked delivery pressure ( ok )
changed airflow sensor and checked all induction. i did have to take head of about a year ago and when putting back together i broke the fuel pressure regulator and had to replace this, and the problem started about a week later.
any help or ideas would be of help,
at your mercy, butcher boy.

If the pump is whining first of all put a genuine fuel filter.

This happens when my fuel reaches the 1/4 empty mark. Cuts out, black smoke, very hard to start... and the pump whines like mad.

If i fill it up to the 1/2 or full mark all my problems disappear. The pressure of all the fuel in the tank pushes it through wherever it has to go and i can drive on problem free.

All i have to do is keep it above the 1/4 empty mark.

hi every one

Have the same problem with my discovery td5. When I try to start, it starts fine for the first time, but cuts out after 20-30 seconds like I am out of fuel. after purging, it starts with the fully depressed pedal and runs ok. this happens when I have less then 20 ltr diesel in tank. in other cases everithing is ok

None return valve in fuel filter head the problem ??

Injector rings and seals or oil in the ecu / injector loom.

is there a vacuum when you undo the fuel filler cap when the tank is only 1/4 full. If so could be a blockage on the tank vent. run the car for a few mins, or even drive a few miles, then switch it off and check the filler cap, it will make a sucking sound. not saying ist defo that but worth a try