It sounds like wifi interference. Try changing your router’s wifi channel: https://support.sonos.com/s/article/3286?language=en_US

Also, refreshing your system would not hurt: power off all Sonos units, reboot router and wait until it has fully restarted, then power up Sonos.

Ive tried channels 1, 6 and 11 and I get the same issue with all of them.

Will try some of the less common channels too but driving me nuts! Everything was completely rebooted last weekend in an effort to fix.

If you can hardwire one of your Sonos speakers with an ethernet cable, you can run your Sonos system in a wired setup, which tend to be more stable than a wifi setup:

https://support.sonos.com/s/article/3209?language=en_US
 

If you decide to run in a wired setup, it is recommended to remove the wifi credentials: 

https://support.sonos.com/s/article/3488?language=en_US

Alternatively you can submit a diagnostic, post the number here and wait for a Sonos employee to investigate, or call Sonos support.

Downloaded AirPort Utility and performed a WiFi scan. Shows that there are a number of networks around me on channels 1, 6 and 11.

Changed to a channel that wasn’t in use (channel 2 in my case) and seemed to be better but still having issues. However, as there's an ongoing issue with Sonos/Spotify playback it’s hard to know whether I’m experiencing the same issue as before or this wider problem.

Will update this thread when the wider Sonos/Spotify issue is resolved - fingers crossed the channel change did it.