Looks like that's where I went wrong, I will try that again (I think the cloud chef builder on the site provides a version based on master).
I did try that before, but I was worried about using an unstable build.
Will give it another go. Thanks for the tips!
...encryption of the 802.11.s mesh interface, I assume...?
Also note that you will need to use a build based on OpenWrt 'master',
in the 18.06 release (and hence the LibreMesh release built on-top)
this didn't work yet.