This one has got me stumped! Looking at a piece of land I shoot on on an ordnance survey map. The next door land is marked with a purple line around and in the middle stands the symbols AL, as the image above! If it had been NT i.e. national trust then it makes sense as they are marked in exactly the same way in purple with a boundary of a purple line around it, but AL?? On the ordnance survey website: Ordnance Survey: Abbreviations on maps
it says AL stands for Alley.