Ok I'm back, shut down computer, modem, wifi, tv, unplugged chromecast and shut down tablet. All back up now and no difference.
Clunky i havent been asked for a 4-digit pin. Basically nothing in the box except small flyer saying go to chromecast.com/setup, but from there i only just downloaded the google home app onto the tablet. I found no pin in the box. Ill go scrounge through the bin to check.