I believe that you have what is called an overriding interest in your neighbours land title. Irrespective of whether it is disclosed on the land title or not (yours is not if it hasn't been registered), some overriding interests are disclosable, when for example, an owner sells up. I'm not sure whether this is a disclosable overriding interest.
It's a pity you've left it for the land to be sold before raising this. If you are trying to do this yourself, I would find out who the seller's solicitor is and contact them and tell them about your overriding interest.