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These are actually walls, Monkeyeyes, and hence no supports as such. The walls do have supporting 'pillars' ,for want of a better word, but these are on both sides of both walls! There are no > marks on the diagrams in what I am calling 'the deeds'. That is actually called a Land Registry Charge Certificate.
There is a marginal note there, Ethel, which reads: "T marks affect the northern and southern boundaries of the land in this title." The problem is that - according to my compass - these are the rearmost, bordering the access lane, and front walls, bordering the pavement. These obviously belong to me alone.
It's not really important enough to consult solicitors, Jenna.
When I moved in over 20 years ago, I temporarily propped a large table against one wall and the old lady who shared that wall lost no time in telling me that it was her wall, that mine was on the opposite side and that I could get my table over there as soon as I liked! I've just always wondered whether she was right and thought of it again now, as the people currently occupying that house are about to erect fence-panels on that very wall.
Thank you all for your input.