so that confirms what i thought must be happening.

when the tank is being filled up, something makes air rise up the filler neck, and that air pushes some of the diesel back up the neck, allowing it to touch the nozzle, cutting it off. this might a vent/breather pipe.

check this out, i got this help from another forum
BMW TIS - 16 11 060 Removing and installing or replacing fuel filler pipe


maybe that breather/filler pipe has got blocked in mine?, which is causing air to rise through the filler neck?

does anybody have any experience or expertise on this pls?