This may be a symptom of network interference or latency between your speakers.

4 meters isn’t very far from your router but if it’s through multiple walls and/ different floors it might be enough to weaken the signal especially if the wifi AP isn’t central and/or if they are connected to 5ghz wifi. 
 

I would move a few speakers or all the speakers to the same room or really close to your router and see if you get dropouts. Move them back one by one. If you notice them dropping out consider wiring the speaker in that location back to the router or adding a wired wifi access point close to the problem area. 

Thanks @Bumper 

I agree that it is probably down to network issues. We’re in a <200sqm and I’m using a Google wifi AP, moving to a FRITZ!Box 7530 AX in the near future with repeater unit as we have a few IoT accessories, so hopefully this will help. 
Looking at the Connection info, Kitchen pair are both on 5ghz, channel 36, SNR 41bD (left) and SNR 31dB (right); Lounge is 37dB, Bathroom 34dB and Bedroom 33dB
The page here suggests that 45+ is good and les than 25 is poor, so I’m in that middle spot it seems.

I can attached some speakers to the network. Does it impact much if I have a couple wired and some wireless? I know it changed how the Sonos network happened with the older kit, however I thought Era worked in a different way 

Era has no Sonosnet so connecting them won’t impact how they interact with your network.

You might want to try switching the speakers to the to 2.4 band because it has better range and for lossless music the amount of bandwidth is not that much.

Also FYI there is an Apple Music bug that was causing drop outs on speakers, even when wired. It was fixed for me through an update I believe. At least my units have stopped skipping. So it could still be this affecting you as there have been some posts about this recently YMMV.

Any one or more wired Devices will create a SonosNet network, according to Recommended settings for using Sonos with Ethernet networks | Sonos

Assuming that is still correct, and I think it is.  A few caveats:

“Wired Sonos products (other than Era models) will transmit a ‘Sonos only WiFi’ for use by your other Sonos devices (Sonos Move, Roam & Era excluded).”  - More info: Troubleshooting Sonos on WiFi | Sonos Community

This is good info also…

Reduce wireless interference | Sonos

Reboot Sonos products | Sonos - if you need to.

Be careful with the SonosNet channel and make sure it is set away from whatever channel your WiFi is set to.  Any wireless Sonos devices that your router reports as wired are likely using SonosNet - at least that’s how it appears for me - most of my wireless Sonos devices show as wired in my router other than the Roam and one member of a Stereo pair (not sure what's up with that - maybe that’s how pairing works).  If you don’t have any devices wired, then a SonosNet conflict is probably not the problem.

If you have not changed anything in your setup, and this problem has recently started, there may be something that changed the networking behavior in the recent firmware updates - impossible to know for sure since there’s no info on what has changed.  Something put our entire system into instability recently - drop outs and other issues. Sonos Support did some remote troubleshooting and restarted all my devices and made a few recommendations that I'm still testing, but it seems better for now.

If you can call manage to get a support call through, they can be really helpful determining the cause of the problem.  Otherwise, you can be easter-egging it forever.