See this link under “Connect to two devices simultaneously”:

https://support.sonos.com/en-us/article/pair-your-bluetooth-device-with-sonos-ace

Yes, done that. I have two devices connected (mobile phone and also a BT transmitter), both connections working.

However, I can not see how to “easily switch between them”, either button presses on headphones and/or from the App:

There are no button presses needed, headphone/app or otherwise.  I quote the link I gave above (bolded added):

When Sonos Ace is connected to two devices, you can play audio from either one. To switch audio playback, pause the audio from one device and start audio on the other.

Yep, seen the support page. It does not work how it is described, it seems to autoselect. If I am playing audio via BT transmitter, and mobile phone rings, the Ace selects the phone, when I hang up, it doesn’t revert back to the BT transmitter.

I was hoping there is an override to toggle the BT source, either from button press(es) and/or within the App. Maybe there isn’t (yet).

I suspect the issue is that your BT transmitter isn’t ‘robust’ enough? 

My experience has been different.  I will use multipoint to connect to my phone and my laptop.  I’ll be playing music from my phone, and then get into a zoom/teams meeting via laptop.   Audio will switch to the zoom meeting AND my phone will pause the music playback. I would assume that the Ace is actually sending a pause command back to the phone, or some sort statement to know that it can’t play audio right now.  When the zoom meeting is offer, the audio will switch back to music from my phone.  Again, there has to be some sort of command from Ace back to my phone to let it know that’s available for music playback again.

I’m guessing you aren’t see that activity with your BT transmitter either because it’s a lower version of BT or the transmitter can’t pause the audio and just ignore these commands.

It should be noted that I don’t think multipoint is perfect.  In the above scenario, I did receive a phone call while on a zoom call, and the result effectively was that laptop ‘forgot’ that Ace had a mic on it.  I had to force the laptop to delete the Ace, and reboot the laptop for it to work again.  I now have a simiar situation, where my Ace ‘Ventura’ is connected via bluetooth, but not recognized as an audio output device somehow.

Thanks Danny. The BT transmitter is a BTA30 Pro, using aptX HD codec:

https://www.fiio.com/bta30pro

What I was expecting from the Multipoint description in the App, was the ability to ‘easily switch’ ie manually select the BT source, either button press(es) on Ace and/or select in the Sonos App. I guess if this has not come up before, it can’t be a common requirement.