Well, the Roam has Alexa built in.  You can speak directly to the Roam to instruct it to play music, no need for a separate echo dot or bluetooth connection.  I think the buttons on the Roam might also be a bit helpful.

If you wanted to stick with your original plan though, you can put the Roam in bluetooth mode.  I’m not exactly sure which echo dots are capable of sending bluetooth audio (transceiver) to a separate speaker, if any of them can, or what the setup steps would be.

There is also the option of setting up an Alexa group, using the Alexa app, that effectively tells your Dot to always play music requests on on the Sonos Roam. I do use this setup with an echo show, because I wanted the benefit of the screen for feedback.   I don’t think it would be necessary for your case though, as the just enabling Alexa on the Roam would be the simple solution. 

Melvimbe - thank you for your suggestions. I tried The Alexa group approach, and while i could get the Sonos into a group with the Echo Dot, i could not get it to operate as a loud speaker for the Echo. It insisted in acting as an independent Echo device. I could not get the Echo Dot to recognise the Sonos via a Bluetooth connection.   I did have the Sonos on Bluetooth discovery mode (blue light flashing) but no joy

Any further suggestions will be gratefully received 

@JohnCa,

In the Amazon Alexa App set the Roam as the Echo Dots default speaker - you need to have the Sonos Skill installed and place the Roam and echo dot in the same Alexa enabled group.

Goto the Echo Dot property settings in the device list. See attached screenshots to help get you started.