Hi Duncan and Fred,
I recently attached two Sonos One SL (2nd Gen) speakers to my network and these are working very well with my B2s. I'm not yet very familiar with the technical level workings but I'm on the learning curve. So I thought that I would put a couple of points to think about here.
As far as I can see, the Sonos app is only required to perform the initial connection of the speakers to your home network. The Sonos app registers the Sonos speakers on the network, gets IP addresses for the speakers and once the speakers are seen by the network the Sonos app can be closed down. From then on, the B2 communicates directly with the speakers. So, in my case, I haven't used the Sonos app since I installed the speakers.
I don't have any Sonos Gen 1 kit, but if it works in the same way then there is a reasonable chance that the Sonos Gen 1 kit will work with the B2.
Brennan don't specify any particular model or generation of Sonos speaker so I would think that you should be able to use both on the B2.
Below is the link to the Brennan Sonos page which describes the workings and also the limitations, but no Generation limitations are given.
Regards and good luck.
Peter.