Finally, after, shall we say an extended period, I got around to attaching my home theatre pc to the television downstairs. The TV is an old CRT so I didn't hold out much hope but to my amazement I have a picture where the Windows 7 Media Center text is readable (I've connected up using a VGA to RGB / s-video converter off of ebay).
The sound works fine through the TV although a little quiet via the stereo, however, time to try the toslink optical cable. Er, yes, a little louder, I could see the cones moving through the mesh on the speakrs, certainly the loudest they've ever handled. Ouch.
So all in all a resounding success.
Just one more job, putting the HTPC into the TV cabinet.
Oh dear, when I measured the cabinet it was big enough for the HTPC case (a silverstone gd01). I forgot hinges! Doh.