Whilst my first attempt is very crude and only delivers a 12v supply, thus suffering from distortion at higher volumes, it gives me confidence to refine the solution over the coming months. 

I got the Play 5 Gen 1 off ebay for £35 and had a couple of laptop batteries lying around which got dismantled for this project.  I'll take better pictures and add more steps/descriptions when I take it apart again to 'upgrade' it.

(The 15.7v is the input voltage to the converter, i have it set to put 12v out to the Sonos)

I seems to draw just over half an amp so with this battery configuration I'm hoping to get at least 3 hours.  It's playing ABBA at the moment so will update this message with how long it lasts. 

With these batteries (extracted from old laptop batteries) i got 3 and half hours before the input voltage dropped below 13v.  It could have carried one but i didn’t want to risk damaging the batteries.  With that, I’m happy the concept has been proven so will look to improve the design, find out where the 24v rail is (any guidance on that would be appreciated) and try to move some of the components internal.

Will update as i go.