Sonos isn’t unreliable for everyone. But it can be unreliable for some users with certain network issues. Are you using a wireless or wired setup? What router are you using?

The vast majority, I venture to suggest. Else Sonos wouldn’t have been so successful. It does however depend on a robust home network. 

I’m an ordinary non tech consumer, sold to by Richer Sounds (who I trust)  I’m wireless, through a virgin router and a Tenda Nova extender system. I have no clue why Sonos will suddenly lose contact with one or all of its products

The answer will lie in how your extenders and router are set up. Your Sonos system is capable of being as rock solid as mine has been for the last 10 years. 

Are your router and Tenda Nova both producing WiFi networks? How is the Tenda Nova mesh connected to your router?

And does the Tenda Nova primary node contain a router, splitting the local network in two? If so, Sonos components (controllers, players) won’t communicate across it.

There’s a lot of info in this thread: 

Sonos is a complex system, totally reliant on local communications. Introduce a complex local network environment without foreknowledge and things can go wrong. 

Thanks guys, I’ve been back through all the Sonos material and the setup instructions make it look very simple. 
I have no idea what a primary node is, and the Sonos system itself gives no clue as to what’s wrong. 
The Tenda is connected to the router by Ethernet (but could it also be Wi-Fi? and splitting?? no idea

Is there a Sonos helpline? 

The link to contact/chat to Sonos Support Staff is shown below...

https://support.sonos.com/s/contact

Your network is set up in a way that makes it impossible for Sonos speakers and controllers to find each other consistently.  You need to address your network setup.

I suggest you try the following

1. Ensure the Tenda Nova mesh is in Bridge mode.  It's in the instructions or just Google it.

2. Temporarily connect a Sonos speaker by Ethernet to your router. 

3. Power off everything. Then power on router then Tenda Nova satellites then wired Sonos then other Sonos. 

See how it goes. 

Yes there is a helpline. See the Sonos Support page 

Thanks John, I’ll try that

Please do seek help from Sonos Support if you need it. Running a multi-device system on a multi-device network requires things to be set up in particular ways and that varies depending on the brand of mesh system. 

For most people Sonos just works out of the box, but when it doesn't it isn't really anybody's fault. It can be very frustrating but can probably be sorted quite easily. 

Keep us posted.