First, the surrounds and sub  connect to your Beam, and not directly to your router. Make sure that somehow the WiFi wasn’t turned off on your Beam in some fashion. 

Second, they would normally only be active when the Beam is receiving a 5.1 signal from the TV set, so make sure you’re testing with such.  They should, however, be working when you’re just playing music, but it’s worth checking that you have the surrounds set to ‘full’ and not ‘ambient’. Well, once they can be seen in the controller, following that first paragraph. 

Please don’t do any further factory resets. They almost never fix anything, and they erase any information that might help Sonos help you. 

Finally, if the previous suggestions don’t help at all, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

Virgin Superhubs have something of a reputation here.

If the setup is being attempted on the Hub’s WiFi I’d suggest temporarily wiring the Beam to the Hub. This should allow setup of the OneSLs without problem.

If on removing the Ethernet cable devices go missing then come back here -- or contact Sonos Support -- for further advice.

I refused an "upgrade" to their Superhub 3 a fair while back, too many problems with it (especially with Sonos).

Ratty, you are a star. Connecting the Beam to the new router by Ethernet cable worked and I now have my surrounds connected to the system. I have been able to remove the cable again which is a bonus! 
 

thanks for your assistance.