Solution found:
https://askubuntu.com/questions/228619/disable-sound-through-hdmi-permanently
sudo sed -ri 's/^(load-module module-switch-on-.*$)/# \1/' /etc/pulse/default.pa
This stops the system from switching output when a device is plugged in, combined with using Startup Applications to set your default audio on power up - I finally have audio working the way I would expect.