I opened the project up again last night, and turned off all the master plugins and it
works fine again.
Looking back, when it was experiencing the no audio issue,
the CPU was peaking, especially on the 8th core (which on my projects usually means the
master bus is too busy), but also it wasn't plugged in to the mains and the battery was
low. Could it have been a combination of bugs, compatibility, power saving etc...
Anyway, I'm still loving Logic X inspite of these little inconsistencies...