See if switching off ‘Private Address’ in your iPhones network connection perhaps improves things. The new iOS feature has been known to cause some connection problems with some routers.

Thanks Ken - I’ll try that when I get home tonight.

I’m afraid it didn’t work. The iPad still works fine, but the iPhone seems to drop connection. I can change the music on the iPad or with an alarm, but the iPhone doesn’t update. It then hangs and says can’t connect. Manually dropping the wifi and back on again does allow it to connect to the speakers and show what is actually playing but it takes a few minutes to get to that point (all devices and speakers right next to the hub, so no issues there).

Try disabling WiFi Assist.

If ratty’s suggestion doesn’t fix it you could go onto also try these two further things:

1. Resetting the Sonos App from within "Settings/App Preferences" you will then need to fully close the App and re-open it and after introductory screens choose the option to "connect to your existing system” (do not create a new system).
2. If the above doesn’t fix it then goto the iPhone ‘Settings/General/Reset’ area and select to reset your devices network settings - this will delete/reset the phones current and previous WiFi network connection settings and reboot the phone. You then need to connect the device back to your WiFi network and check its its connection is working by perhaps opening a web browser and seeing that you can access an online site. Then try the Sonos App again to see if it now works correctly.  

I clicked best answer by mistake. Unfortunately removing Wi-fi assist didn’t resolve the issue.

App reset didn’t work either. Trying on a older iPhone. And also doing the Wi-fi reset now. 

Ken - Wi-fi reset on iPhone seems to have worked fine. Will test it more tomorrow when the kids are not asleep but thank you for your help so far. 

That’s okay, I (& others here) owe a lot to @ratty anyhow  - glad the WiFi reset seems to be working, but as you suggest, let’s see how it goes for a day or two.