Yes, it will be fixable in most instances, see if this support link assists you to quickly resolve the matter:

https://support.sonos.com/s/article/5097

Thanks for advice but all my products were/are already on S1 so not sure downgrading is relevant. Nonetheless, I have reset one of the speakers but essentially the S1 app has become useless. All options are greyed out so I can neither remove or add anything. 

When you goto the list of your devices in the S1 App ‘Settings/System/About My System’ … what version of software is shown next to each device in the list? Are any showing version 14.14 (build 69132100)?

The speaker I reset I managed to downgrade but neither S1 or S2 is allowing me to add anything, the option is greyed out and both apps are asking me to update the software. I have tried this many times but it fails (error 9) every time.

Why can't things just work and Sonos leave things alone, I now have thousands of pounds worth of speakers that don't work. 

If you are still struggling I suggest that you power down any S2 compatible devices and uninstall the controller. Make sure that there is no S2 controller installed. Critical data is stored in the players, not the controllers. If no players are wired, temporarily wire a player and reinstall the S1 controller. Be sure that you are not setting up a new system (otherwise music service registrations and SONOS playlists will be trashed), you are setting up to control an existing system.

After the S1 system is running, then you can power up the S2 compatible units.

Yes 5 of the 6 are. The ceiling speaker is showing 11.5 (build 571231030)

Then I have 2 that are flashing green lights having been reset and can't add them to S1 (option greyed out) one has been downgraded in S2 the other can't be added to either app 

You might end up having to wire the speakers via LAN cable temporarily to get them back on S1.

Sounds like someone's accidentally upgraded to S2 via the app.

This is a massive failing with the Sonos software imho, only one master account should be able to do this.

The fact they are showing the 14.14 version, unfortunately means, at some point, accidentally or otherwise, they have been updated to S2 - you need to factory reset those that have been updated and then use the S2 App to downgrade each ‘reset’ device. As per the link I posted earlier.

Importantly - Do not reset the older Amp that’s running on S1.

Once you have used the downgrade tool on the ‘reset’ products, you should then remove the S2 App.

Next, Open the S1 App and you should see your Amp only - once you are in that position, you can then add back your reset/downgraded devices, one at a time, until all are back up and running on your S1 HH.

Pretty sure I didn't and I have expressly advised Jnr not to upgrade either. 

Not going to throw blame but finally got there, reset and downgraded everything and re-added through S1. 

As you say Keith this is a huge issue, the irony is that the more speakers you have the worse it is. Resetting & relisting 7 speakers is a nightmare. Add to that pairing and TV setup. Sonos really should not allow that to happen so easily unless they want to upgrade my old speakers for free! 

Exactly the same thing happened to me - half of my system just upgraded itself to S2 overnight! Resetting and downgrading to S1, and then re-adding to the system, was nowhere near as painful as I thought it would be. Took about 10 minutes per speaker.

Let’s all just remember what an utter engineering fail and arrogant joke it is that Sonos is wasting our time like this