Post Processing Woes [Solved]

I've hit a brick wall with post-processing. I can't get it to do anything at all. Followed all the tutorials, set up the layer and volume (global ticked), and nothing makes any difference. I'm obviously missing something pretty obvious here.

Unity 2021.1.17f1 + built in rendering + latest Post Processing. Camera set to deferred.
I can select anti aliasing mode in the Post Processing Layer, and that seems to work ok, but no effects set in the Post Processing Volume have any effect.

 

Reboot time. That old chestnut.