Did you try the options here?

https://support.sonos.com/en-us/article/connect-sonos-to-a-new-router-or-wi-fi-network

Yes I did.
And I can’t complete that because I appear to have entered a doom loop where the factory reset apparently won’t complete.

Have you force closed and restarted the controller? You could also uninstall and re-install the controller.

Yes, I’d previously uninstalled/reinstalled twice and got the same result.
Just tried the force close and reopen, but the app is still saying a factory reset is needed even though one (well, five actually) has appeared to complete successfully. 

Try temporarily attaching an Ethernet cable and then go through the reset process again. 

Ah,sorry…..I forgot to say in my initial post that I’d tried that as well.
Thanks for the reply.

During Factory Reset you can release the button after the status light starts flashing orange.

If there are any other SONOS units on the network, power them down.

Is there any error message during the update? Sometimes I notice that an update proceeds very slowly. Initially I’m thinking there is a failure, but I’ve learned to go make a sandwich and when I return to the system the update is complete.

SONOS support has tools that we can’t access. You’ll probably need to work with support. Phone support is best.

So….to sum up. Having changed broadband provider, I can successfully add the speaker to a new network with an ethernet cable connected but no way can I enter the wifi network details to make it work as it should.

Not for the first time the app lets me down on the basics.

As always with Sonos….great speakers, risibly crap software. 

No, no error message. No other Sonos units.
From the instructions I’ve seen, the factory reset is complete when the flashing orange/white is replaced by the flashing green. Is that not correct? 
I think you’re right, I’ll phone tomorrow, although I can ill afford the time.
Thanks for your help.

As above, I'm thinking you only hold the button until you get the orange-white flash and then let it up. It will finish the reset, then flash green.

Sorry, but no.
No difference whatsoever.

The size of this community and the number of issues increasingly posted on here illustrates how badly designed and implemented Sonos software is in the first place. Given the cost of these, admittedly good, speakers…..it’s almost criminal how Sonos has let its customers down with the rubbish software they peddle.

Some folk have experienced update issues if they’re going from a much older version - it’s too big a jump to update in one step, but Sonos support has provided interim updates to help. Might this be the case here? Either way, contacting Sonos support seems the best course of action. 

Thanks, yes I’ll be phoning them later.

I think I had this setting the wireless on a new device recently because my phone wasn’t using the 2.4 freq on the router?

In some cases there is a router setting, maybe the router default or after an update, that isolates 2.4 and 5GHz segments. Transactions will not crossover. The SONOS system might be split between the segments. SONOS units must be able to chat with each other and the controllers. As a result, because the phone signs on to one segment, SONOS units using the other segment will be stranded. Later, the situation could reverse. If there are multiple phones, they could be split between segments. This can give rise to “my” phone works, but “hers” does not. Often in these discussions there is a claim that the “WiFi is perfect”. This is only partially true because web browsing and email go straight out to the Internet and there is no need to interact with other local devices.