the non return valve you see in the lft of the image is from tank to primer bulb. i was working on the assumption that the valve inside the primer was letting fuel drain back.

the hose with the green arrow is coming from the engine to the fuel filter as a return line for un-used fuel. it returns to the tank unless its cold then then the warm diesel goes back into the fuel filter to prevent freezing..
thats what the nissan workshop manual says anyway.

so far i haven't had the issue of cant start since fitting the one way vavle before and after the primer, cleared the dtc of p1615 and also fixed the loose hose clamp.
still early days yet.

it was doing it with
full fuel tank and low tank aswell
on hills pointing down and up.
engine cold and hot.

im hopeful this issue is gone but if it returns i will update this.
if its fixed then others might benefit from this.