I am not in a hurry to sell but on the other hand I don't see the point in asking for an unrealistic price only to drop it later. I usually reckon that half the original buying price is a good guide for any secondhand item in good condition.
It's probably a good thing you're not in a hurry, the potential buyer is going to need to be specifically looking for such a bike. If they are, then your original price looked good value, half the new cost with light use is a bargain. As mickeg point out, if someone is looking for a bike with S&S couplings, it adds a lot of value, but it adds nothing if they don't require them. That potential buyer might be reading this now, or it might take ages for them to come along, but the fact Thorn are still selling some of these, indicates there is still some demand.
How much is it worth to someone not looking for such a bike? Impossible to say, I wouldn't consider spending much on the wrong bike, I doubt many people would spend serious money on something they didn't want, however big a bargain it was. Knocking a couple of hundred quid off, isn't going to change that.
Good luck with the sale.