I'm posting just to help anyone else who may have the same problem - I've seen a number of similar issues in this forum.
On trying to add the second disc to to a brand new three disc whole home system no location was suitable, even right next to the first disc.
I tried factory resets, two different android phones, but nothing worked.
Using https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer I saw that my BT homehub and the whole home disc were using the same wifi channel so I tried turning of the homehub wifi by logging in to it at http://192.168.1.254/ using an ethernet connection from my computer.
As soon as it was turned off my phone was able to verify locations and I was able to install the the other two disks without any problems and the system is working perfectly now.
