I'm a property lawyer in England (are you in Scotland, think I saw something suggesting that on another thread) and the market is incredibly slow at the moment and people know they can push for a bargain.
Ask to see a copy of the valuation to see what has downvalued the property from your expectations and ask the agents for their advice, maybe general property market, maybe something else you're unaware of.
There are also a number of sites on which you can check recent property prices for comparatives (again take into account the state of the market).
http://www.nethouseprices.com/
http://www.houseprices.co.uk/
If it's in an area with a good rental yield (eg near a hospital of college or university) you could try to find any interested investors to widen the market.
If you still can't shift it for the price you want, could you rent it out until your early repayment has gone to give yourself some additional income?
It may also open the market more for investors who want a tenanted property with a good rental yield to add to their housing stock and they may get a more favourable buy to let mortgage than normal buyers if the rental figures are good.
Might be worth having a chat to some local letting agents about possible figures and need for rental properties.
Do you know if you have any options on your mortgage such as payment holidays or if there is any room for manoevre on the early repayment figure, especially if you own other property.
Make sure you get a full illustration if you ask about any of these options though so you know exactly what you might be getting yourself into.
Good luck :)