Are you saying your iPhone is freezing or just the Sonos app?

Just the Sonos app, it’s not freezing at all, the services are blinking and the search scrolls back to the top every time I try to scroll down to select a result from a service not in view

Try deleting the Sonos app. Reboot your router. Update your device. Re-install the app. 

What’s the theory behind this approach?

It’s basic troubleshooting vs factory reset of your speakers which should only be done in extreme circumstances. Therefore you follow the path of the least invasive procedure. You first want to look at your network, and make sure all apps and device software are up-to-date. 

Have you tried simply rebooting the phone?

And I’d also suggest the theory is a bad install/corrupt memory in use by the app. Deleting and reinstalling the app, especially after a potential reboot of the device your installing on might clear that corrupted install. 

In this case I don’t think that rebooting the router will be productive. I’ve seen this sort of issue with other applications on my iPad. Sometimes reinstalling the App is helpful in the short term. In the case of the SONOS controller nothing is lost and reinstall with a fast Internet connection takes seconds -- much less time and trouble that rebooting the router. I’m speculating, but I think that there is an issue with one of the underlying elements of iOS and Apps using this element are at risk. I’m not seeing this sort of issue the Android’s or PC’s.

On the network side of things SONOS is far more ‘chatty’ with other network clients than most applications. A web page, email, Twitter, etc. has only one network client -- the external Internet. The SONOS controller App, on the other hand, along with external music services, is constantly chatting with the SONOS players and the players are chatting with each other. This requires a stable network setup.

That said, I cannot rule out a router issue. What is the router model number? Maybe a case against this model will develop. Many years ago I had a router that needed to be rebooted at least once a month. The Internet was on fire with flames about that router. This was an ISP supplied router and the day I was able to replace that router the nonsense stopped. With simple systems, such as a computer or two, the router was OK, but for complex systems such as SONOS, thermostats, lighting, and control systems, the router was a mess.

in any case, refrain from Factory Reset of any SONOS units without further consult.

This post IMO has gotten out hand. Not to say that my suggestion is the be all to end all but IMO the OP is not willing to try a few simple steps to resolve the issue. This is simple 101 troubleshooting. It you have  an issue with your computer most often a reboot resolves the issue. Now if you need to know the mechanics behind a reboot then I suggest taking a computer programing course. If the reboot does not work then you look further.

Thanks, yep, going to try this when back. Rebooting the phone made no difference. I know iOS sandboxes everything so will delete, reboot, reinstall and hopefully that will help