Yet another non start issue

Hi, I'm running out of ideas so any help would be greatly received.
Engine cut out while pulling up to a junction (I thought I stalled it but no). Engine cranked but wouldn't fire. I turned it on the key to move out of the way. AA came out and couldn't figure out the fault so left. Got towed back home.
Checked: ECU loom all good, had injector seals done 500 miles ago.
Fuel pump replaced. Fuel filter housing replaced. Replaced non return valve. Inertia switch working. Battery fully charged. Fuel sucked out and replaced. Immobiliser working.

I'm concerned I can hear a funny noise coming from the cooler when the pump is priming? Could air be getting into the cooler?

Once the engine is running it will seem to run all day, turn it off and then won't start again. Very frustrating.
Any help greatly received thank you.

Hi, first of all tell if it's Disco(manual or auto) or Defender

Hi sorry, its a disco manual

Then first of all check the crank sensor's connector and wiring as long as you can reach it, do you have a scanner to watch the rpm while it cranks and doesnt start? if not replace the crank sensor to rule it out... also check battery voltage while cranking, if it drops below 10.5V it will hardly start and probsbly you need a stronger battery... eventually when it refuses to start do somehow to bump start it without cranking and if it starts right away you'll have to replace the starter motor cos it has an internal failure which drops the voltage and disturbes the crank signal.

an intermittent hiss sound in the FPR area while it's purging is normal, means that the HP of the pump is probably working