after my last post I did some more digging and found something that seems to make it work properly!
see here - https://wiki.ubuntu.com/Audio/Installin ... verModules
in the "Install with Command Line" section, copy and paste the 3 commands into terminal and then reboot, my sound is now working as it should, i.e. through the speakers which then cut out when I plug in the headphones (without having to add/change anything in /etc/modprobe.d/alsa-base.conf)
hurrah


