I have a 2003 VW Passat 1.9 TDI (130bhp) that starts fine in the morning runs fine but when warm and left for 5-10 minutes requires about 3-5 seconds of cranking over before it starts again, then it fine again.

Check the radiator it might be losing water or stuffed and the water does not has normal flow
And take a look at the water pump as well

I have a 2003 VW Passat 1.9 TDI (130bhp) that starts fine in the morning runs fine but when warm and left for 5-10 mins requires about 3-5 seconds of cranking over before it starts again, then it fine again.
Do you mean: 1.8L ?
That sounds more like a fuel delivery problem than coolant flow.

I recommend you take it to the dealer. The fuel system is under pressure and shouldn't be tampered with if you aren't familiar with such systems.

The problem may be a faulty fuel pressure regulator or possibly a failed deceleration shut-off valve.