Hey @LiamG, welcome to the Sonos Community and thank you for your post!
Sorry to hear that you are missing your Sonos One SLs in your system. A quick look at your account tells me that they are certainly present, as in you did not create a separate system.
Did you set them up as a separate room or added them perhaps as rear surrounds to the Beam? In the later case, they would not show as individual items in the app, but be part of the Beam’s room.
Should you have added them as a separate room and a different controller (another mobile, desktop or web app) wont list them either, please reach out to our technical support who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.
Hope that helps!