mid-terrace
^ I, perhaps wrongly, assumed you had to do it along with width of the walls, not just the short bit accessable from the front and back.
Presumably your neighbours heat their houses? I would imagine most of the heat loss is to the outside world. Does the dividing internal wall even have a cavity?
@NickCJ started
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^ I, perhaps wrongly, assumed you had to do it along with width of the walls, not just the short bit accessable from the front and back.