Have you checked those speakers are indeed on Sonosnet (WM:0)?

Yes, all speakers are WM: 0.

The availability of speakers in Spotify is inconsistent. They are probably accessible 80% of the time, greyed out the rest. 

I see you are in the UK. Do you have a BT Smart Hub?

https://en.community.sonos.com/troubleshooting%2D228999/bt%2Dsmart%2Dhub%2D2%2Dconnection%2Dissues%2Dapril%2D2021%2D6857959

Things work fine using the Sonos app?

@106rallye - Thankfully not BT! Linksys Velop tri-band (connected to FTTP).

@Kumar - yes, Sonos app on Android and Windows works fine with Spotify. Main reason for using the Spotify app is I can shift to my phone and back if I'm moving around at home.

I use Spotify Connect because I prefer the native app and very occasionally I see this.

Have you tried reinstalling the Spotify App? And rebooting Sonos? Don’t do both together though, or you won’t know what worked if either does.

Reinstalling the Spotify app will mean losing music downloaded to the phone though...