For the record:

Here is what worked round it.

Switch off wifi cured the ‘reboot coming out of standby,but not. A solution for a Smart tv!

Switched  to separate wi fi network because there are some reports of NAS using Bonjour causing similar problems ( boot after standby) so I tried a clean internet connection outside my home LAN . This did not cure the problem.  The github site which has a lot of android developers talking about  various issues, also mentions Sony tvs rebooting after standby and measurements taken during standby of tv behaviour.There some pointers at HDMI causing partial wakeups from standby as well as some ( not applicable here) issues if VPN clients are installed on the tv.

So after a bit of trial and error around these factors:-

Cured. Switched off Bravia sync on Inputs menu and disabled listed devices In Bravia sync while leaving them physically connected to selectable HDMI porta.

Now tv coming our of standby normally.

Conclusion. Probably a design problem. There are mentions of the problem for several years back.

Reason for the tv working normally before update. On initial  installation I set up tv aerial and internet only connections with no live HDMI connections. Later I connected Apple TV and Fire tv to HDMI. On input list they appeared as HDMI and the labels I had attached. All worked fine.

The HDMI inputs were live during the update. I had noticed that there two new Streams on the inputs list marked from the Amazon and Apple Devices. These showed too in Bravia sync. As i say these devices were connected during the update and subsequent self  configuration probably switched on Bravia sync.

So my problem appears solved, related to updating with external HDMI being live during the process causing them to be configured in a way that then caused problems. I guess in some ways they caused the tv to misbehave in standby, communicating over the wifi.

Im posting in case this helps others if they have similar problems.