Hi,
The April edition of Classic Bike Guide had a buyers guide and listed the Dragonfly as having a value of £4000-£6000. Personally I feel this is a little high and have seen them sell on eBay for £3000-£4000 in recent times. Obviously condition/originality and other factors will play a part but if you're looking to sell then I'd try selling it on here first and then eBay. Alternatively you could try getting in touch with the London Douglas Motorcycle Club who I'm sure will have someone in your area.
Rgds
Andy