First, ensure that your mobile Sonos controller device meets these system requirements…

https://support.sonos.com/s/article/4875

Then perhaps try temporarily wiring one ‘standalone’ Sonos device direct to your primary router and power-cycle your ‘other’ Sonos products, one at a time. Next open the Sonos App from a fully closed state and try running the update again (check updates at least twice to ensure all devices are updated) and then when successful uncable the wired device.

If the Sonos app itself is unable to update from the relevant app store that suggests a fundamental internet connectivity problem. 

Thanks for the help.  I have switched off all devices apart from the boost which is obviously hard wired to router.  It is needing an update so tried it. No difference, same message, 

“ Something occurred during your download, please try again.” 

All other router connected devices, TV, Central heating control are all working fine with no downloading issues. 

I wonder would I factory re set all devices individually ? 

I would perhaps first try another ethernet cable with the (power-cycled) Boost linked to different ports on both the Boost and router.  If that doesn’t work for you then just set the Boost aside temporarily and see if wiring another standalone speaker in its place then gets the updates for all your Sonos devices. Then perhaps post back here with what happened. I would not factory reset anything at this early stage.

Many thanks Ken , I’ll give that a try

I’d also be looking at other potential reasons why your Sonos devices can’t reach the update server. VPNs, virus protection apps, etc. 

I concur that a factory reset would not be a solution.