Can you confirm that you are selecting the Beam within settings and then selecting Add Sub?

The system is set up in a room with beam and two surround sounds (ones) so can't specifically select the Beam

Please open the Sonos App and select the cog wheel in the top right of the screen, then select the Sonos Room containing the Beam.  Please see screen shot below, are you selecting the ‘+ Connect Sub’?

Ah sorry. The sub is not showing in the app at all so no have not tried that is if I do it just says it can't find it. 

Everytime i factory reset and try to add it I just keep landing on the attached screen that it may not be appearing in the system. But it never shows up even after I plug out and plug back in.

​I've tried it 10+ times and have never got to the add to room screen

If I go in to network settings it does show up there but even updating the network (to the same one it is on) it still is not appearing in the app for me to add to my existing room.

Try connecting the Sub Mini temporarily by Ethernet to your router.

Once the Sub Mini shows in the app, add the Sub Mini to your Beam, then disconnect the wired connection.

Thanks both for your help. In typical Sonos fashion having faffed about trying to get it to connect via the ethernet with no joy, when I tried again to connect via WiFi it worked perfectly. So got there in the end. Thanks