Thanks for the reply.
I have no doubt I can mirror cast my android phone to my TV, but that's not very useful when I need to take a call or receive a text as I often get called by work and want the show to continue for others. It's also tough on the battery and the phone would likely need to be in a charger. So unfortunately that is not a solution for me, I need the chromecast button within the NOW TV application to work as it does on the original chromcast.
I cannot playback content on my android phone because it's not a registered device and as I said earlier I am at my maximum number of devices (four) after adding the latest chromecast ultra.
If I play first before casting I get "the maximum number of devices has been reached error". Which is exactly what I would expect. I cannot make any further changes to my devices list until December 1st according to my account page.