Found the problem – apologies for the panic.
During the upgrade, the “dbus” daemon had not been restarted. I don’t know why, but this meant that the avahi-daemon could not restart properly, which meant that although the mt-daapd server was up and running fine, avahi wasn’t broadcasting that fact, so I couldn’t connect to it.
All sorted with “invoke-rc.d dbus start”.
Cheers all,
CC