Some progress.  I now have Mythbuntu 9.10 working on at least one of the TV input channels. It seems to work very well, with easy to navigate programming schedules.  The first real test to to record a couple of films this evening (Control and 24 Hour Party People as it happens - I have a long-standing affection for Joy Division and Factory Records). I've also got it daisy-chained from the Humax box, and the S-video output comes into the TV on a separate channel from the Humax box. The big challenge now is to get the remote control working, so I don't have to run it via a vnc connection from my Linux notebook.  More later