Unfortunately, the 5GHz radio does not support mesh - as per these posts:
I have tried the solutions suggested in those posts, but none made any difference - the mesh interface would never come up. To be honest, this is the least of my worries, I can come back to fix that later, I've spent too long this evening trying to fix something that really is optional.
Overall the mesh now works - that is the important bit.
I'll post my full config for the sake of others once I'm finished.