Discovered this yesterday after 6 years of L494 ownership - not sure all specs/models have it but it was very useful whilst waiting for a delayed train at the station!

It's not available (touchscreen key is blanked out) when engine is on or if you turn on the ignition with engine off.

The only way to access it is to press radio on button (volume knob) and when screen comes on you then press "climate" button and then the "rest heat" button is available.

It stayed on for 25 mins with engine off yesterday and saved me from freezing! 2014 L494 V8 s/c Autobiography Dynamic