This for info to others in same situation.
I wanted to add a WiFi AP to my Vodafone router to get better coverage in my home.
I had an Ubiquiti unifi AP and so installed that. Install went fine. I used SSID and password that the router came with for the unifi AP to give seamless swapover.
However the router would not switch to the AP or back again unless I forgot the network and rejoined every time I moved around in my house!
If I changed the Unify AP to a different SSID and password it would refuse to changeover without manual intervention as well.
As a last resort I changed the default Vodafone SSID and password to something different and then changed the AP SSID and password to the same. Immediately it started working correctly with the the wifi changing over to the AP completely seamlessly as it should.
