Your description of the status LED colours etc; seems to infer the Sub Mini is rebooting and maybe overheating - see this link: 

https://support.sonos.com/en-us/article/sonos-led-lights

I would perhaps collect a few diagnostic reports before/after the Sub reboots and maybe a video of what you’re seeing and then return to Sonos Support - they may go onto suggest a factory reset, but I would just wait and see what the Staff say, as a reset may wipe out important diagnostic data.

You might want to see if anything changes aswell, if you run all on your WiFi signal, instead of wiring the Beam to your router.

Thanks for the reply, I wondered about over heating but it was running for a couple of hours prior to using the TV 

This issue has happened when running both via WiFi and also connected via ethernet which was suggested by sonos support today. 

I do wonder whether there is something wrong with its internals as the only other thing it could be is the most recent update but that would surely affect everyone 

I suspect MINI hardware failure. Make sure that the power cord is firmly seated.

Power cord definitely firmly seated 

It's also interesting it sits on standby for 18 hours until you use it then it loses connection. 

It's also annoying that sonos support said they can't Identify anything wrong with it.

Maybe giving them some further diagnostic reports will help them to identify the cause - it’s clearly intermittent and perhaps something will go onto show up in the data. If convenient, try the Sub elsewhere in the room aswell, connected to a different power-point.

I captured some diagnostic info from last night so will call sonos back again.

I tried moving it to another location and it still dropped out and did the same 

Really seems to me like it's had it to be honest 

Sonos support this morning suggested that I connect the sub to the beam via ethernet is that something that's possible as not heard of that being able to be done ? 

Temporarily wiring the MINI might result in additional diagnostic data capture.

If you had some sort of severe wireless problem, it should show up in the diagnostics. Were you able to capture a diagnostic while the unit was operating normally both idle and playing, and another after the fail?

Am I correct that one TV App seems to trigger the failure? If so, bracket starting this App with diagnostics.

Still having the issue. Sonos support just keeps asking me to do checks over and over again. Nothing is working it's also dropped out when not even playing the TV app so seems unrelated. Also I have wired the TV via ethernet and turned its WiFi off. 

Going to call the support line again tomorrow and basically do my best to tell them they need to replace this faulty sub. 

Nothing else in the network sonos or otherwise has any issues only the sub mini 

Literally just watched it drop off the network and go to a red light whilst connected to the beam with an ethernet cable 

I’m just wondering what happens if you remove the Sub-Mini and Surrounds from the Beam, and go onto make a ‘stereo pair ‘with the One SL’s and then bond the Sub-Mini to that stereo pair - does the Sub-Mini then also dropout in that type of 2.1 audio setup?

Don’t forget to record the test-steps taken and grab any diagnostic reports along the way too, noting the reference numbers, as any evidence gathered may later prove useful and perhaps help the Sonos Staff to identify the root-cause.

Thanks for everyone's help with this.

Called sonos support again today and they have decided that they are going to replace the sub