Exit: It's a mind boggler for sure. Since the original post I have gone a whole week with no problems. I hadn't driven the car this weekend so this morning I started it and it fired up first time. I then realized I'd forgotton something, went back in the house, came back out and it wouldn't start, absolutely nothing and it stayed like that for 20 mins. During that time it would not start in neutral. It finally did start and off I went, no closer to knowing the issue, although at least I now know it won't start in neutral.

One thing I have noticed is a small diesel leak from the pipe that i think goes into the Diesel Fuel Thermostat. (I think that's what it is, see picture)

Could that be it?