If you have tried the downgrade with the Sonos connected via Ethernet and it still failed, It may be defective, I’d call support and ask them to look at your system.

@PJSmyth 
Did you cable the Play:1 speaker to the LAN? If not, perhaps see if doing that will work to downgrade the Play:1.

1. Reboot the local router (recommended)
2. Cable the Play:1 speaker to the LAN/router
3. Delete or simply reset the S2-app and all data.
4. Reinstall the S2 app (if not installed already)
5. Factory reset the Play:1 speaker that you’re downgrading.
6. Open the S2-app and chose option to downgrade the wired device.
7. Note: there has to be a S1-only product visible on the network too.

Here are links to both the factory-reset and downgrade instructions:

• https://support.sonos.com/en-us/article/reset-your-sonos-product
• https://support.sonos.com/en-us/article/downgrade-a-sonos-product-from-s2-to-s1