Bugger me! I was blaming my phone as it always appeared when I sat down to watch TV - which was off course when the Chromecase was switched on - I even posted to XDA about it https://forum.xda-developers.com/android/help/los-14-phone-started-to-kill-router-ip-t3728044
Curse you Google - I went and bought a new phone (OK, not full reason, but this played a part)