Hi Y'all,
Thanks for all the suggestions, and "yes" the zones can be heards from each other, so they need to be "tight".
I have tried several different methods & found only 1 which worked (and didn't cost ££ extra).
(belana sound / volumio / dockers / with DACs and without, many failures!)
Default raspbian & Snapcast is the way to go...(using the "apt install" versions from the standard repositories, not the latest, but they work!).
Very simple mechanism, basically a broadcast across the network to multiple targets (er...multicast? No, multiple unicasts I think, TBC).
It's the mechanism which Volumio uses, but being a commercial org now, they've locked it out of the user's hands & charge for this "premium" feature.
Volumio is great, very reliable, pretty, easy, but for stand-alone I think.
So basically any source is attached to the snapserver as a sound device, and all the snapclient devices use the stream as a sound source & the local sounds device as the target. (VLC used to do something similar).
So pi-zero is fine as a snapclient, even with a HiFi DAC board.
Ditto for windows as a snapclient (snapserver for windows isn't a simple download, it might be compiled perhaps, but not by me).
Ditto for an Android phone as a snapclient sending to bluetooth.
Also pi2 is fine as a snapserver.
In my case I exported the prepared playlist from MC27 and saved all the MP3 files in a folder.
Popped that folder onto the sdcard inside the snapserver, told MPC/MPD about the files, made a playlist & bingo!
It can be controlled by an MPD client on an android phone, so skip forward, back or choose next song etc.
Might even be possible to use it as a DLNA renderer, with snapserverfifo as the audio device? (not tested).
So thanks to MC27 for helping make the playlist & me massively tidying it up so it was a consistent MP3 source.
All ready for the party now!