As I remember…Loosen the bottom nut up toward the mirror. Spin the mirror maximum into the perch. If the angle is wrong, unwind until correct. Then tighten down the bottom nut. Then tighten down the top nut.
I'll try and get some more pics. Idk if it's ceased but I can't undo it. It's also tricky as I have to use adjustable spanners due to the size. Kinda wish I'd bought the massive Halfords tool set instead of which ever one I got.
Very likely seized into the thread. It happened on the XJ. I stripped the mirror AND the perch when removing from each other, so replaced the mount with one from a Chinesium mirror mount from Crapazon, and new mirrors.
As I remember…Loosen the bottom nut up toward the mirror. Spin the mirror maximum into the perch. If the angle is wrong, unwind until correct. Then tighten down the bottom nut. Then tighten down the top nut.