It’s a One, not a Play:1 that you own? Play:1 is older and doesn’t support Airplay. 
 

Have you enabled iTunes/Apple Music in the Add Music Services part of Settings (cog icon at bottom right of phone app). 

Thanks – ah it is actually a Play:1… but I’m sure I used to play my iTunes through it? 

I did try to add iTunes but I don’t have an Apple Music account so I couldn’t do that.

So as its a Play:1 is there no way to play the library of music thats stored on my phone – only streaming services?

Some years ago there was something like “on my phone” but Apple changed something and stopped it. That was prob what you were using. 

You can use the Music Library facility if your iTunes music is on a NAS or hard drive somewhere?

Upload all your music to iBroadcast.  It is a free, cloud based storage for your music library.  iBroadcast can then be added as a streaming service in your Sonos app.  Works great.