Hi dorsetdave, try the below: 

-Unplug the TV from the mains and restart your router.

-Update the TV:

https://www.sony.co.uk/electronics/support/oled-tvs-android-a80j_a83j_a84j-series/xr-55a80j/software...

-Test on a personal hotspot connected to your TV instead of Wi-Fi. 

-Perform a full reset: https://helpguide.sony.net/tv/iaep1/v1/en/08-01_03.html?hl=factory

Of course, test after each step, let me know what happens.