@goddesshunter; thanks. However, the thread is for a mirror with a button directly attached to the glass as per Series 1 whereas my R3 has the rain sensor (as far as I can tell, the mirror attaches to the back of this not the glass) and I can't find a scredriver hole.
The sensor cover may be hiding the hole and I need to get the 2 plastic halves off to find it.
If I don't get any more info, I'll be introducing those 2 plastic bits to my old friend, brute force. . . . . and I'll update this with the results for those that follow.
231 with 400 bhp single-turbo 13B REW conversion
Beware - older people may not be better drivers but they can afford better lawyers