First of all you must have done more than just change your password for WiFi. You must have also changed the SSID  as well. How long has the speaker been moth-balled?

A while. I can’t remember exactly but definitely a few months or more (it’s a bedroom unit and I just haven’t been listening to music / using music alarms in there for a while).

I have had to factory reset my router a few times which resets the SSID however I always change it back to the same one. I do have the SSID hidden and use MAC filtering (yes the Play 5 MAC is white-listed) but that hasn’t been an issue before.

Might be easier to wire the PLAY:5 temporarily to your router with an Ethernet cable, get it updated, and once it’s up to date, then move it to wireless. 

Try moving closer to your router for initial setup. Once setup you can move it to the desired location. One more question… do you have range extenders or access points in your network?

That’s going to be a right pain given my setup (it’s mounted on a stand that it’s locked into) but I guess I’ll give it a go !

Since you are moving closer to your router consider @Airgetlam suggestion.

The Ethernet connection did result in an update. 

Afterwards (back on Wifi, connecting on iPhone) I got as far as the “Join Sonos network” phase but it can’t join said network for some reason...

That suggests that the network card maybe failing in the Play5. Suggest you contact Sonos tech support. Normally I’d say run a diagnostic but since the speaker is not on your network doing so may be for naught. But you can still run it and post the reference ID in your next post. 

Bizarrely it let me run one earlier : Ref 657478373

Figured it out in the end. It turns out the wrong MAC address was whitelisted - which is really weird, maybe I made a mistake whilst typing them out during the last router reset.

Oh well, it’s working fine now. Thanks for all the suggestions!

You’re filtering by MAC address?! Forget it. It’s of precisely zero use as a security measure.