There are a number of specialist legal eagle practices in this area - you will generally find them in the rural towns in the Shires of England. Here is just one (picked at random).
http://www.balfours.co.uk/index.php?option=com _content&task=view&id=186&Itemid=42
I'm not quite sure what you are after here - presumably an anaysis on the probability of success? The ONLY way you should contemplate buying this property is of the basis of the agricultural tie market price plus a very small risk premium. You need to ask yourself why the owner hasn't bothered getting the tie removed.
The process of so doing involves active marketing the property as an agricultural tied house for at least 6 months, after which lack of success may enable the owner to have the tie removed. You will have much more difficulty if the property is in a National Park or AONB because the planning authorities are more precious about 'second homes' and folks from the cities buying places in the country, forcing the locals out.
Also find out as much as you can about the provision for social housing around that area - the policies and the current provision versus the demand. The 2 are not automatically linked but it stands to reason that poor agricultural wages tend to mean that agricultural/forestry workers are more likely to be housed in social housing. If there is a dearth of such properties, it is likley to be more difficult to get what you desire.
Is this linked to your recent 'derelict house' question?