In the first instance you should establish if the land is registered or not by sending an index map search to the Land Registry by using Form SIM (enclosing a map of the area), they will then tell you definitively if it is registered. You also download that form from the LR website by going to "Forms". Call them to see if a fee is payable, it wasn't previously but think one may be now £5.00 approx).
It would be better if you had enclosed the land, the application would be much more likely to be successful. Given the length of time using it you may still have a chance.
If the land in question is unregistered then you must have been using it continuously for 12 years in order to claim possession of it and be registered as the registered proprietor at the Land Registry
If the land is registered then you need only have been using it continuously for 10 years in order to apply for adverse possession at the Land Registry. Notice of the application will be sent to the current registered proprietor of the land, who is obviously at liberty to object. It is therefore more difficult to claim adverse possession of land that is registered.
In the case of both applications you would have to submit form ADV1 to the Land Registry together with sworn statutory declarations regarding how long you have been using the land (from you and others who know of your use) and it may be something over which you should consider taking specific legal advice from a local solicitor to ensure that the application and supporting documentation is drafted correctly.
I suppose the owner is likely to object in this case and you may get sticky and expensive if you instruct solicitors to act your behalf (which you probably should).It's obviously nearly impossible to predict how much legal fees would be if he disputes you litigate but they would be unlikely to be less than £2, 000.00 as a minimum. If you think you can get a better price by simply asking him then do so to gauge his position.
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