You won't get away withthe adverse possession plan- probably not, as you have not exclusively used the property adverse to his original ownership. also you will make enemies doing this, land grabbing, as it can be known, won't always appealing to the neighbours... it's best to try to reason this out. google his name or find out anyone who knows him what his address is... put a strong case to him that you can getthe council to acton the derelict nature of the place (if this is possible) and then ask theat you two reachan amicable solution (you buying it is one good option, if you can afford to...)