Hi, just to close the loop on this, now I've owned the car longer, and being in winter, I actually think it's to do with the smart alternator. What I have found, even if the temperature is below 10 degrees Celsius, if the interior heater is set to off, the revs on starting are normal, circa 600rpm. Switch the heater fan on and the revs raise to 1000 rpm until the engine is warm then automatically drop back to 600 rpm.
If on top of the above you also engage the viscous heater, it will raise the revs to 1200 rpm, again until the engine is warm then automatically drop back to 600 rpm. If you switch the heater fan off at any point before the engine is up to temp, the revs drop back to 600 rpm. The viscous heater doesn't have any effect unless the heater fan is running.