Rocking door mirror on E38 735i

[gf15]

Mirror is on a 98. It is loose, I removed the little black flap underneath and the post seems nice & tight. It pivots back electrically & all seems well. Everything seems to be as it should from underneath.
Is there a collar on the top of the post (like behind the mirror glass) that comes loose ?, if so how do I get at it to tighten it?
Thanks in advance