ah right, you didn't say that in your fist post, it thought it was a mystery who it belonged to.
without sounding too cliched, it's worth what someone will pay for it...which means you'll have to ask the solicitors that own it. there's no such thing as 'market value' in the current climate, it's whatever someone is prepared to buy it for.
if you want to see comparables (which means nothing really at the moment) use rightmove.co.uk and search under land within the postcode.