No throttle on take off

Good evening Disco lovers, i bought a 99 TD5 from my father in-law and have since replaced the head gasket (along with all other necessary gaskets), replaced coolant hoses, replaced radiator with a 3 row aluminium radiator, removed fuel tank to check if fuel lines from tank to filter housing were damaged (all ok, checked as there was a lot of build up of moist dirt attracted to fuel filter and housing), and now once you start the car (everything is ok) until you put your foot on the throttle to take off and the revs don't pick up at all (even when the pedal is all the way to the floor) but still just idles. (hopefully that makes sense. It doesn't matter if you leave it in park or whether it is in drive. after a minute or two you place your foot on the throttle slowly then the revs start climbing (again doesn't matter if your in drive or neutral). Once you take off and the car drives as normal all is ok, just seems to happen after sitting for a while.
If anybody has encountered this problem or have heard of it please help me out.
the fuel tank had been emptied for removal so my thoughts are swaying towards the high pressure side of the pump (as the low pressure sends more than enough fuel to the head) Please correct me if i am wrong.
Injectors have new O'rings, they have also been placed in their original position and adjusted to manufactures specs. The head has been cleaned and pressure tested with no problems whatsoever.

Unplug the maf sensor and try again but leave the engine at idle for a minute first

Hi. What about the red ECU plug? was it checked for oil ingress? cos in all the work you've done you didint mention anything about the injector loom even though this symptom seems more like a TPS signal issue... For a clear diagnostic a live data log would be relevant