There are some steps required that you haven’t mentioned that you’ve completed.  Can you confirm:

1/ You’ve added your Spotify account as a music service in the Sonos App?

2/ You’ve added the same Spotify account to your Amazon Alexa account?

3/ You’ve linked the same Amazon Alexa account to your Sonos account when enabling the Sonos skill within the Alexa App?

4/ You’ve specified Spotify as your default music provider within the Alexa App?

5/ What command are you using to play music?  Try ‘Alexa, play Supertramp’

Thank you!  Yes, I’m pretty sure I’ve done all of the above and I can see things I’ve played on Spotify in the Alexa app.   Re point 2. We use a family Spotify account with 5 users so the Spotify account linked to Sonos may be using a different Spotify account and email to the account I use as part of the family group that I’ve linked to Alexa.  Could that be an issue? 

You need to setup each family account as per: https://support.sonos.com/en-us/article/use-multiple-accounts-for-the-same-music-service and then switch between them within the Sonos app under source.

Thanks for trying but even though we’ve now added all each Spotify user to Sonos Alexa still cannot find the service to play.  Looks like we’re just going to have to make do with weather updates only from Alexa.  Spotify will not be discovered to play.