Hi Peter!
I'll offer a middle-ground suggestion between jags and Fred's and ask if a complete reconfiguration of the bike might result in more use.
Sometimes, repurposing a bike can make it seem entirely new and will result in more use than imagined. I'm not sure how yours in configured, but swapping 'bar types (straights or comfort for drops or vice versa), changing postures, choosing a new direction (say, a semi-porteur with front platform rack or basket for urban shopping) can make a world of difference in how one views and uses a bike, and for little additional investment. The option to sell always remains, though in this case, things are in rare alignment for you to move the bike on elsewhere.
Lots to think about, and I've been there myself. 'Trouble is, one takes a big hit on depreciation compared to the price, so one has to figure the difference is the cost and pleasure of enjoying it. I've sometimes thought about moving-on one or another of my fleet, but when I considered how little I'd get for them, I declined.
Very best of luck on your sale if you go that way, and -- yes -- I'd suggest hitting the classified boards before eBay. Best way to reach a motivated buyer. I'm about to do the same in selling a 25 year-old car, figuring clubs are full of people who are already interested in the marque and know what they want and I'll likely get closer to true market value without having to give eBay a commission. Have you thought of posting it to the Thorn Forum private-party classifieds? Sometimes people are willing to wait a bit for delivery if they can get what they want from a known owner and at market value.
EDIT: Maybe the timing will be right for a good match:
http://www.thorncycles.co.uk/forums/index.php?topic=7945.msg51749#msg51749All the best,
Dan.