Did you set up the Wireless Audio Sync feature on the Apple TV? https://support.apple.com/en-us/HT210526

I did, that seems to make it worse, so I reset it back to how it was. It’s not a major issue but very off putting when I’m watching a film. 
 

it’s so strange, it works perfectly on my other Apple TV in bed bedroom. That’s a cheap TV. The tv in the living is a new LG smart tv. 

Are your One SLs currently paired as a stereo pair? If so, try unpairing them and doing Airplay to only one speaker. Do you still experience the lip sync problem? If not, while the audio is still playing, group the other speaker in the Sonos app and see if they both stay in sync. This isn’t an ideal solution but it could be a good alternative.

Thank you so much! That’s actually helped. There’s only the minimalist of lag now, and it’s much more bearable to watch. 

Perfect lip sync over Airplay isn’t technically feasible (as Sonos just gets an audio signal with zero control over video), but its good to see you minimized the delay to make it usable for you.