The last thing you should do is recognise any ownership if you are intending to make a claim on the land because you have used it as your own for over twelve years (squatters rights). However, if you have used the land with the owners consent or paid them rent then you have no right to buy from a private individual. Why do you now need to do this?
you have a right, but only if the owner is willing to sell - you cannot force anyone to sell just because they have been kind enough to let you park there for many, many years rent free