I installed the latest alsa drivers v1.0.23 at the time. This didn't fix it, but it may be part of the solution. The final piece of the jigsaw was to add 'options snd-hda-intel model=dell-m6' to my /etc/modprobe.d/alsa-base.conf, create /etc/modprobe.d/modprobe.conf and add it there too, then restart.
02:00.1 Audio device: ATI Technologies Inc RV710/730
00:1b.0 Audio device: Intel Corporation 5 Series/3400 Series Chipset High Definition Audio (rev 06)