Diesel turns over but won't run

Ok this is about a tractor so it's slightly OT, but it is a 4wd one so that counts.

This is a Fiat/NewHolland 80hp tractor with a 3.9l Iveco normally aspirated 4 cyl motor. No electronics no computers no nothing.

Normally it just runs and does it's thing, reliability personified (or should that be tractorfied)

Yesterday went to start it, tuns over but no start, normally it starts instantly. Plenty of juice in the battery, plenty of fuel at the injector pump, cracked open an injector pipe, plenty of fuel.

Fiddled at bit more, suddenly it roars to life. OK who knows.
Drive off. then 150meters later it dies again that's when I noticed that the lights on the dash were off.

Still cranks OK but no go.

So it's an electrical thing but what?
God I hate auto electrics!!!
Secondly I always thought that these simple motors just kept running once they were lit so long as there was fuel?

Any ideas as to where I start looking would be much appreciated.