Thank you Ben.
I first checked the volume controls on the ZP100, and they track with the controls in the Sonos app but not in Roon. This is with the latest versions of all software, so I just assumed two sets of volume control were in play.
Similarly, I’ve just taken a look at controlling the Sonos with Roon and the Sonos app. If I change the volume in the Sonos app, the volume displayed by Roon does not change, but the other way around - controlling the volume via Roon - does change the volume in the Sonos app.
I guess I’ll live with it until I can replace some of the Sonos players.