trig42: ... Failing that, is ther a device that takes HDMI in and splits the audio out (ie, HDMI in, Coax/Optical and HDMI out)?
This looks like just what you need:
http://www.dealextreme.com/p/1080p-4-port-hdmi-input-to-hdmi-optical-coaxial-audio-output-converter-100-240v-ac-16553?item=2
I've bought (cheaper) stuff from DealExtreme before, but it is caveat emptor!
I wonder if the Pi HDMI interface is the same as an XBox 360?
http://www.dealextreme.com/p/gold-plated-1080p-hdmi-cable-analog-optical-digital-audio-for-xbox-360-24868?item=16
Something else I came across while looking for this might be of interest to other Pi-ers:
http://www.dealextreme.com/p/hdmi-v1-4-male-to-vga-female-converter-adapter-cable-white-15cm-130458
Currently I'm waiting on an el-cheapo version of the same thing (will be interesting to see if it works):
http://www.aliexpress.com/snapshot/110490931.html
I do trust AliExpress - excellent service. Trouble is they often require quantity purchase. (Not this time - but an Xbox 360 converter from Ali, only $1.65-odd, but minimum purchase 150 :-(. )
:-)