The two plastic shells that surround the rain sensor and mirror support come apart quite easily as they just snap one into the other.

Then you rotate the mirror support 1/4 anticlockwise and it just comes out from the metal disc that remains glued to the windscreen.

After removing mirror plug and rain sensor plug the rain sensor is removed by pulling the two plastic rods that hold it over the coloured plastic lens that also remain glued to the windshield.