LG C3 + ARC = atmos audio drops

Hey there I am hoping this thread picks up a fix but having read so many others I’m not sure.

Firstly I have read similar threads to my issue but feels that its worth posting to see if this is a different issue.

I have just bought a LG OLED65C34LA and connected it to my 4yr old Sonos ARC, configured the sound to run bitstream, pass through and it works fine when not Atmos, but when Atmos the sound isn't stable, I get a drop every few seconds.

The issue is on native tv apps (Netflx, apple TV) and also when I connect my apple TV 3rd gen to the TV where its different with Apple is there is sound until I play Atmos content and then nothing.  Sky is similar with Atmos audio very unstable.

I have upgraded all HDMI Cables, I have reset and rebooted countless times on all devices. all software is up to date with LG releasing an update this week that has not resolved the issue.  Also connected devices at different stages to see if the order and pace is a factor and it isn't.

So any thoughts?  I am poised to take it back to Currys (UK) and they are already saying its a Sonos issue so that should be a fun experience, but I would sooner fix it at home.

Thanks in advance to anyone willing to take a look at this post.