Thanks for the offer but I don't think so. And after a week of bumping I've finally had an answer over on AVForums.
The S480 and S580 are supposed to support the .mkv container. Which they do via a USB. A first for Sony. But I was after a definitive answer as to whether they supported .mkv over a home network like eg the WD TV Live. They don't.
It appears I'll still have to wait for the perfect one box solution for under my TV.