thanks for your responses guys, they are all helpful.
I'm not getting any leakage from anywhere, it 100% ends up in the overflow bottle. Regarding sucking in air when it cools down, I understand the point, but by the time it gets to cooling down it is already over red rover. From cold, the system when it is entirely full, radiator bleed valve bled so the radiator is full to overflowing, it all warms up, and air/gas is forced into the coolant, pushing it out into the overflow bottle.
It is definitely happening at the hot stage, not afterwards, and there is not a drop dribbling out anywhere it shouldn't be. In fact when I siphon it out of the overflow tank and then top up/bleed the system, I'm back where I started, everything full to where they are meant to be. There is no overall loss of coolant. This is why I think something is forcing air/gas into the coolant under pressure.
I went and saw a radiator guy yesterday, and he also had the standard hand held sniffer that changes colour in the presence of carbon monoxide. Problem is that the car really has to be under load for there to be sufficient pressure to pump lots of air/gas into the coolant. Idling produces so little that the hand held sniffers don't pick it up. I live in the Adelaide Hills(SA) so I'm up and down hills every time I drive.
He has pointed me to another radiator guy who has a $20k testing machine, including carbon monoxide, so that is my next step.
I agree with the comment that head gaskets are generally not a problem in a VZ, particularly one that has just done 140k. But.... my first mechanic rang a mate of his from a Holden dealership, who said he was doing 2~3 head jobs a month on VZs because of this problem.
Anyway I did learn one very interesting piece of trivia that explains what insane person would ever place a thermostat behind the engine in an impossible spot. Turns out GM bought the engine design from SAAB (remember them?) where it was intended to be an east-west motor, and where the thermostat could be accessed in 10 minutes. GM turned it north-south.