I’d first attempt running through the TV Setup option in the Sonos app under the soundbar’s room settings.

You can also check in your TV settings that you have Bravia sync enabled.

Do you have TV Autoplay turned on in the app, under settings for the room?

Ho thanks for your replies. I contacted Sonos Support and after they run diagnostics it turns out the HDMI cable is dying. 
 

Autoplay in the Sonos App, apparently isn’t an option yet. It may appear in the next app update. 

Hi Rhonny, where in the app do you access the menu that you have taken a  screenshot of? I can’t find it in the app. 

Rhonny, cancel that I have just found it. Though when I Googled it, I did read that Autoplay wasn’t available on the app. Thanks

Does that solve the issue? (I can’t believe an HDMI cable can “die”, as suggested by Sonos support.) 

Autoplay was already turned on. 
Yeah I thought it was strange advice,  re the HDMI cable dying. I haven’t changed the cable yet. I will see what happens when I switch the tv and system off and then restart it tomorrow. 

Did you run through the TV Setup in the app? That should identify any issue with the HDMI connection.

Mr T. It seems to be working fine now.