mk4 golf tdi strangeness

[Big G]

ok so the golf is a '98 1.9 TDI 110bhp version with 274k on the clock and still the original engine. Never had a head gasket gone/replaced afaik and I've known the car for 150k+ of it's life.

Drove it 20 miles tonight and stopped to get a takeaway. it's gonna be half hour so we decide to pop to my Dad's to wait and then return. After returning to the car it won't start. it turns over like a petrol does without spark plugs... Tried several times to start without any luck. Tried locking it and unlocking it. still nothing.

Cambelt is intact and everything is spinning as it should when turned over. I removed the mickey mouse clip off the to top of the fuel filter and turned it over again and saw fuel coming back from the pump.

Since it was already hot I ruled out heat. fuel returns from the pump so can practically rule out the fuel. the only thing it must be is compression right? Car drove perfectly before.

so my dad turns up to tow me home and we decide to roll down the hill I'm pointing down to see if it'll bump start and it does... so we turn the engine off again to see if it'll start off the key again. it won't. bump it again and it starts grrr. so we stop next to the tow car to demonstrate it turning over and the bloody thing starts first time!

So I drive back to dad's and park on the hill just up the road from him and try to start it again. it won't start off the key lol.

time to investigate in the daylight tomorrow now.

Any ideas?