Are you using the TV App or Sky Q App for Netflix?  Netflix on a Sky Box doesn’t support Dolby Atmos.

https://www.sky.com/help/articles/setting-up-dolby-atmos#:~:text=Dolby%20Atmos%20is%20not%20available,such%20as%20Disney%2B%2C%20Netflix%20etc. 

Thank you for a prompt reply - I had tried watching output through the apps on the TV rather than through the SKY puck but without success either.

..and sadly redwine didnt in this instance help!!

@Nigelg52 Set the TV to these settings:

HDMI Input Audio Format: Bitstream

Digital Sound Output: Pass Through

eARC Support: On

Then play the film The Tomorrow War from the TV’s native Prime Video app and tell us what audio format is displayed on the Now Playing screen in the Sonos app.

Thank you GuitarSuperstar !  I had seen your responses to others with the same issue.  However I cannot find where the suggested settings are on my LG TV !!  If I go into the Advances Sound settings screen none of what you have very kindly suggested appear and further if the HDMI lead is connected to the TV from the Beam most advanced options are in any event greyed out.   Cannot see how I might send you a screen shot to better explain or are you able to send me any visuals?

I think that this is the correct online manual for your TV - http://kr.eguide.lgappstv.com/manual/w21_mr6/global/Apps/w6.0_mr6_e04/e_eng/settings.html# 

HDMI Input help is in Settings>Sound>Select HDMI Input for Audio Format

Digital Sound Output help is in Settings>Sound>Digital Sound Output

eARC Support help is in Settings>Sound>eARC Support

Please note that a lot of these settings are only available if you have set ‘Optical Out Device/HDMI(ARC) Device’ under Settings>Sound>Sound Out>Use Wired Speaker

Take photos of the audio settings on your TV and post them here by tapping the photo icon:

Thank you!  These are screen shots of the settings I have available to me with the Sonos Beam plugged into HDMI port 2 (labelled as e arc on the side of the TV   As you can see the dont match up to your suggestions

using their supplied cable

If you scroll down in the advanced settings, you should find the settings GuitarSuperstar has mentioned. 

Doh!!   No I feel very silly in that I had not picked up on the scroll down option!  Thank you guitar man and outburst for both your patience and help on this.  Greatly appreciated - odd that neither Sonos nor LG support teams could achieve this outcome.  The expression teaching old dogs new tricks springs to mind!!!

Of course what I failed to mention is it now works brilliantly and to be honest had not realised that Sky Sports etc is also in Dolby Atmos.  For the info of future users - Atmos output DOES work thru the Sky Puck and the streaming channels thereon ie Prime, Apple etc so dont even have to use the LG remote to access the native apps. RESULT