Restricted performance but no fault codes Diesel

Hi all, despite having my third S type I am a new member here hoping somebody can help with this annoying niggle. As there are no fault codes I can only think it as non-electrical... When I run the car with a light right foot it runs fine but as soon as I add power it comes up restricted performance... it’s the same when starting the car if I go through the procedure of turning the ignition key on and off two or three times as if priming the filter after change it runs well for a fair amount of time ...I seem to remember the Ford C Max did this when it needed a fuel filter change... So I change the filter and even though it’s better it’s still doing it. I have been told by a diesel fitter who owns the range Rover with the same engine that sometimes the pick up pipe in the tank can get blocked ...It just seems like there is a fuel restriction somewhere...does anybody have any ideas? any help would be greatly appreciated, Regards Jon