Read this and hire a solicitor. The land will need valuing as it sounds like you have a ransom strip of sorts.
http://www.propertylawuk.net/propertytransacti onsoverage.html
this bit:
The on-sale of the land in its present state, where the vendor fears that the purchaser may take advantage of a rapidly rising market to make a quick profit from the land. This will be particularly useful in the case of a vendor who is anxious not to be embarrassed by being seen to have sold at an undervalue, such as a local authority or charity.
You are
not a charity. Get it valued. Unless of course you are also housing authority, in which case, if you own say 50% of the land? then maybe you're entitled to 50% of it's worth.
Best of luck in your quest :-) and get the link copied for yourself and your solicitor.