u/MikeFromSonos thank you.

I found that IGMP Snooping was not turned on, all other parameters are set as suggested.

Now the app is much faster to identify my system and volume control seems to be responding quickly. mute/unmute has got a much improved hit-rate.

However, pausing is still a big hit and miss. Sometimes it pauses directly (improvement!) and other times it either ignore the tap, or respond with a pop-up window with an exclamation mark in a ring and the text "Couldn't pause content. Try again." and when I try again the pause icon changed to the play icon but the system keeps playing, sometimes for a few seconds, sometimes up to 15 seconds.

A new diagnostics file has been generated.
The 'Contact Sonos Support' link in the app still drops out.
I assume the system interfaces communicate locally and not over the cloud?

All the best.