Spluttering - lift pump?

I've had this problem on and off for months now and still haven't fixed it... Driving me mad!

It'll be fine one minute, the next the engine will start to die, spluttering etc before it eventually dies! It didn't do it for ages, until today! I've had all the lines checked, new lift pump and new fuel tank!

So I know the pickup pipe shouldn't be blocked?
I've been undoing the bleed bolt (been replaced and retapped so uses a solid bolt rather than a banjo, does this matter?), when I undo it there seems a lot of pressure/air! Then I've been firing the engine up and revving it until diesel comes out, should I be pumping the lift pump instead?

I have noticed a diesel leak coming from the copper pipe that bolts onto the tank on the passenger side, would this do it?

Any help would be great because I'm going out my mind lol!

Tank on passenger side is it a military set up of some sort as UK models have tank under drivers seat?
The bleed bolt will be fine so long as its got a sealing washer
The biggest problem by far with any diesel starting fault is air getting in to the system.
Normal way to bleed it is open bleed screw and pump the lever on the lift pump, but with the 200tdi they self bleed pretty quickly, also pump may not work as its on cam.
If you can see a diesel leak its a good chance air is getting in
The very best way is to replace the fuel line tank to lift pump then its totally eliminated, maybe change lift pump as well then its all new.
Theres 2 pipes, larger one is the feed the smaller one is the return to tank.
You can get plastic nylon air brake pipe from any motor factors, you only need 2 meters as its a short run