I suggest that you refrain from Factory Reset without further consult. As you have discovered the reset does not resolve fundamental issues, but it also destroys diagnostic data that could have been useful.

While the FIVE’s are wired, make sure that the WiFi credentials are correct.

Further testing reveals that everything works ok if one of the Fives is wired via ethernet, however units will no longer connect directly to 2.4g Wifi. So Sonos Wifi is the workaround and I have had to order a Boost.

System worked fine connected to my house Wifi for several months until recently...

I suspect that someone changed the WiFi setup. With a wired player, SONOS ignores WiFi.

There may have been an update sent to your router whilst you were away, which has then changed a setting? 

A possible hypothesis...but...all other Wifi devices in house still work and I have a large number of smart systems including Philips Hue, Home Assist, Alexa, Raspberry Pie, Grid Connect etc. etc.

I have also reconfigured and rebooted router network and factory reset Sonos Fives and tested the issue from different mobile phones (both Android) and the app fails when resetting the Wifi settings to the speakers.

My working hypothesis is that either an update has borked the Android app on my mobile devices or that a firmware update to the speaker has borked the Wifi setting process from the phone app to the speaker.

Nothing critical is stored in the controller Apps. You can reset or uninstall the Apps without trashing the system.

If you have not already done this, “reserve” IP addresses for all regular network clients.

We cannot rule out a hardware issue with one or both of the FIVE’s. As a test, make sure that the FIVES’s are not paired, power down one of them and work with the other, both wired and wireless. Do the same with the other FIVE. While it’s unlikely that both FIVE’s would fail in exactly the same way at the same time, Mother Nature is never fair.

While it is unlikely that there is an issue with a SONOS firmware update (else we’d be flooded with complaints) you should check for updates while the FIVE’s are wired.

Check the WiFi credentials that the FIVE’s are using. 

At various steps along the way, submit a diagnostic, log the confirmation number and keep notes about the configurations along the way. We don’t have access to the diagnostics here, but you can work with SONOS support.

As I have around 100 network connected devices...I’ll be leaving IP assignment to DHCP.

I “solved” the problem by installing a boost - $149 to fix the issue, cheaper than my time to debug,

Anyway thanks for your input.