The solution to my extremely hard to start ZD30DDTi.

FUEL PRESSURE RELIEF VALVE!!!!!

It took about 35 mins to swap over and now it starts like it was brand new.

For Bosch rail 0445214112

Part Number 1110010018

I got mine from United Fuel Injection in Perth ph 1800 199 701. Give them a call, they're very helpful and friendly, and will post anywhere in Aus for a small fee. And you know you're getting GENUINE BOSCH. Much cheaper than buying a whole new rail.

http://www.unitedfuel.com.au/

So if your Nissan Patrol has progressively gotten harder and harder to start then this would be the second thing I'd look at. The first thing to check would be the primer bulb, if the valve in that has gone your diesel will be draining back into the tank. You're Patrol will start easier if you park it up nose down.

Thanks to Rumcajs for all your help.