Hello JonDavies. I am not sure if you have resolved your question yet. But, just in case, here are some ideas:

If you can get access to the actual speakers to confirm what goes to Red and what goes to Black on the speaker connectors that may give you insights. You may also be able to confirm that specific model of speaker and determine the speaker impedance (e.g. 8 ohm). 

I had an application at my place where I had small diameter wires (4) going to each of my speakers (from an earlier set-up from the previous owner). I combined two wires for connection to the Red terminal and two for Black (ensuring I stayed consistent on the matching).

If you are not quite sure on what goes where, you may also try connecting a nine-volt battery to individual pairs of the wires at your termination area to see which combination causes a “bump” on the specific speakers. 

You can connect up to two pairs of 8-ohm speakers to the Sonos Amp (in parallel). (Three pair if they are the Sonos by Sonance speakers.)

To add additional speakers to a single Sonos Amp, you can introduce impedance matching volume controls (one per pair) (with the specific setting for the impedance protection based on the number of pairs), or a speaker selector box with an impedance protection feature (with or without volume control). Sonos would prefer you to buy additional Amps though, and that is another option that would give you the ability to play different things in different “rooms”.

Hope you have success. You will enjoy it when it is all in service.