Does anybody have any idea (preferably with idiot-proof instructions) about how the rear-view mirror re-attaches?

I was cleaning the inside of the windscreen today and knocked the rear-view mirror which just popped off.

I've tried several times to re-attach but I'm obvious missing the knack!

I've got the main mirror and the bit of plastic duct that hides the wiring loom, I assume it twists and locks somehow but any pointers would be great.

Cheers,

Alex