I'm glad I did ask about removing them as I would have snapped the mirror glass if I'd tried prising it out from the unit...anyway, pictures...
How to remove E30 mirror glass (this may only apply to facelift cars)
Tools you'll need: Small flat bladed screwdriver.
and if you're removing the unit, and 8mm socket/spanner to remove the unit
First, get your mirror (on or off the car):
Next, you need to locate the hole in the underside of the casing, it is in right about the middle of the mirror unit.
If you look into this hole (you might need a torch) you should see a little grey/white tab (just about see it in my picture above). If you don't see the tab, you'll need to level off the glass (press it in at the lower edge gently). Insert the screwdriver into this hole and onto the tab.
Gently prise it to the tip to the right (handle to the left) and it'll click/pop.
Once popped, push the top of the mirror into the casing and it'll come away from the back of the casing.
This is the back of the mirror and shows how the glass attaches to the casing, it is currently set to 'locked'.
Here is the picture of it 'unlocked'.
Simple enough design really and works nicely. Refitting is the reverse of removal to be honest. Place the glass back in, slide the tab to the left and you're all set.
