Its hard to say.
Glastoveteran has imported a MY07 roadster for £18500 (although he's having problems with his importer).
If your car is not selling, then its too expensive. If you've had it 6 months then its probably depreciated by at least £250 a month in that time too. If I was you I'd be chopping money off the asking price depending on how desperate to sell.
Another thing I would do is stick it on ebay starting at about £14k, reserve of about £18.5k or so and see what happens. Ebay is a great place to get exposure for your car, i sold both my zed via eBay.