OK, had the same issue with my new Estate. You have to set up the default positions. I went to 2 dealers before one of their mechanics found out how to do it - it's not in the handbook! Here's how:
1: With the ignition on and the car in neutral, adjust both mirrors for the 'normal ' driving position.
2: Check the 'mirror down' function on the conmputer is selected 'on'.
3: Engine off, ignition on, car in reverse, move the mirror swith to 'L' and adjust the mirror so you can see the kerb;
4: Still in reverse, turn the ignition off - this stores the 'dipped position.
5 Select neutral;
Job done. Now when you have the ignition on and select reverse with the mirror switch in 'L' position, or select 'L' when in reverse, the mrror will dip to the stored position. When you take it out of reverse, it stays dipped, and only returns to 'normal' when going forwards at walking pace.
Good luck trying the above - it's not obvious is it?