On the LG TV press and hold the configuration button. On the sound menu make sure following is set:

Sound out: HDMI ARC

Additional settings:

DTV Audio Setting: Auto

HDMI input audio format: Bitstream

Digital sound out: pass through

eARC: Enabled

Try those and see if it fixes it.

Ok, I will have a look. Thanks

Thanks for that. Amazing it has worked. I would never have figured that out in a million years. 

No worries, enjoy! If you have an apple device or can borrow one, I recommend running trueplay :)

I have run Trueplay but I guess now all the settings have changed I best run it again

Thanks for your help

@Simmz Sorry to bump, but just registered to thank you for posting this  I’ve had my Beam Gen 2 for 3 weeks connected to LG Oled and whilst the sound was adequate, your suggestions  have TRANSFORMED it  

I now get much better sound, clearer dialogue and even Atmos ( we watched Whiplash last night - oh my)

Thanks again, glad I found the thread

** EDIT - see resolution below **

I have the same issue and I have tried the above settings but no joy.

Using LG TV’s internal speakers, when I open Apple TV, Netflix, etc I get a banner telling me that it is both Dolby Vision and another one saying Dolby Atmos. When I route sound through the Arc, it tells me that it is Dolby Vision *only*.

I have set the settings below, but the lack of an on-screen “Dolby Atmos” sign makes me think that something is missing…. Any ideas or else is “Dolby Vision” what I should expect?

Additional settings:

DTV Audio Setting: Auto

HDMI input audio format: Bitstream (I have to have the ARC’s HMDI selected otherwise it is greyed out - which makes sense, I guess as that is the only HDMI which is an outbound data rather than inbound)

Digital sound out: pass through

eARC: Enabled

** Resolution **

It turns out that although my TV now only says “Dolby Digital” on-screen and there is no mention of Atmos, when I go to the Sonos App, it says “Atmos”, so I am guessing that it is all ok?

Yes, that’s ok. The TV only displays ‘Atmos’ when the internal speakers are used. The Sonos app will show Atmos when it’s detected, so that’s your only guide really. 
it would be great if the LG TV’s displayed Atmos and maybe a volume number, but we’ll have to wait for eARC2 for that…!

I’ve tried the settings above but still only get Dolby Digital 5.1 showing in the Sonos app (like others I’ve proven atmos is there as it’s selectable when routed to internal speakers)

Any thing else I could try?

Answering my own question but might help others, turned out to be a Sky Q box issue. Used apps on TV instead of through Sky and now get Dolby Vision and Atmos. 

Sky Q box doesn't support Dolby Atmos on any of the apps, it only works for their own content. Box doesn't do Dolby Vision either, just HDR.