Martyn,
These are worth a look
http://www.walkerbay.com/gallery/dinghy.php
Not completely inflatable but you can get the collar to make them extremely stable.
You can also add a small outboard.
From a sailing point of view I would think a little more sprightly than the tinker tramp.
Car top-able too I would imagine.
Google on the name and there are dealers everywhere.
Robin.