Thanks, SkylineUSA. I've had the radiator checked (it's only a year old anyhow) and the pump is showing no signs of failure. I've replaced the radiator fan and the auxiliary electric fan circuit has been intentionally shorted to keep the fan on all the time.
It's a kind of bizarre problem, actually. The temp gauge just sits higher than it's usual mark under normal conditions, but I can watch the oil pressure gauge and get an idea of what's happening. In heavy traffic or if I turn the air con on, oil pressure slowly falls until it's virtually zero at idle, as if the oil is getting really hot and losing viscosity. Then the temp gauge will start rising a bit, and the car will start running like a pig.
I don't think the fuel economy can get much worse than it is now - I get under 300kms to a tank.
I will run some cooling system flush through the system and pop out the thermostat and see what happens. Worse thing is I might have to pop it back in, but I have lots of gasket paper and lots of time
In any case, I will shortly order the NISMO thermostat.