No water in the oil, no steam coming out of exhaust, not losing water (until it is pissing out of the overflow bottle) water in overflow and radiator is green and not gluggy (all seems very normal).
Original thermostat did not seem to have any score marks suggesting that it was not getting stuck or sticky - seemed to be in working order.
We changed all coolant/oil/filters when we imported it back in 03 and have never had any problems with it since - but it has never been taken out and flushed properly.
Temp only goes up after a bit of a flogging (it is an auto so this doesn't take much). Then once you stop it boils out the overflow bottle. Whilst driving with temp up you can hear gurgling noise coming from the engine from inside the car.
Everyone I have spoken to is totally stumped!