Its called Advantageous Possession (squatters rights) and the period is generally 12 years, but the rules are not straight forward. The law is a little vague about who did what and when. You can't expect a council to inspect every square inch of land it owns as many who thought they had squatters rights over council owned land have found.
In our case it can be shown that the Council knew and acquiesced to the annexing of the land, it even gave planning permission to move the fence in 2001, but it is not shown on the title deeds and so the position is not as clear. There may be some condition somewhere that the land can be used but is liable to be cleared if the council so decides. A very unsatisfactory position as this represents over 20% of the plot. the house is great though and just what we want.
Mike.X