Since the Play:5 (Gen 1) is only compatible with the S1 app, you would need to upgrade it to the Play:5 (Gen 2) or Five to be able to play it together with the Roam or any other S2-only compatible speakers.

Sonos has an Upgrade Program where you can get a discount for upgrading your speaker:

https://www.sonos.com/en-us/upgrade

Thanks, i notice the Play 5 2nd Gen is unavailable (in white). Is that normal, or might it signal an updated version?

The PLAY:5 of either generation is no longer produced. The ‘replacement’ is the Sonos Five. 

The Play:5 (Gen 2) model is discontinued. Sonos produced a white Gen 2 that had a white outer shell and a black speaker grill:

The Five is available in all white though:

Hi @LochyHood 

Welcome to the Sonos Community!

You can absolutely switch between the apps. You could have the Roam on S2 alone, or have the Play:5 on S1 alone. In either case, you can run both apps on one phone and switch between them at will. 

If you’ve already set up the Roam as a new S2 system, you’ll need to factory reset your Play:1s/Sonos Ones to get them onto the new household (system).

Only if you want to be able to group all of your speakers together to play as one do you need to upgrade anything. If you’re happy with using two apps, that’s completely fine and you need spend no money.