Just coming back in on this and really asking if there are other Mac users out there who have experienced similar problems or maybe have connected with no problems.
The reason for asking is that after several frustrating attempts to do this myself following the advice from both Raymond and Daniel, (software updated, thank you both again), I contacted Sonos. I spent five hours back and forth with an online "expert" whose advice was basically, "unplug everything, reboot and start again". I finally got to talk to a specialist in the US who tried everything via remote access to the Mac, but eventually said that the problem was that the Mac would not recognize the B2. There's no problem with wi-fi strength, he ran an independent test. He said that the instructions on the Brennan website seemed to be for a PC and not necessarily a Mac and suggested contacting Brennan to see if there was a "workaround" for a Mac.
Maybe relevant, maybe not. The Sonos tests to connect to local music folders on the Mac all worked and the FE Explorer Air Drop I phone app connecting the B2 to Sonos works too (albeit really erratically), so the problem would appear to be solely that of connecting the Brennan to the Mac.
I bought the B2 because I wanted to play FLAC quality through my Sonos 1 speakers, as although I love their sonic quality, it's somewhat wasted on the standard MP3's on Apple and Amazon streaming services.
Any help or shared Mac to B2 experiences most appreciated.
Thanks
Mick