There is no shared lock, no shared door.
There are two flats in the building. The lower flat comprises the ground floor of the building and has both a front and back door from outside (it's for sale just now so you can go view pictures, search For Sale 53 Ardbeg Rd) both of which access ONLY the lower flat. The upper flat is over two floors, comprising the first floor of the building and the attic floor of the building and has a single door from outside, at the rear of the building, which is into a stairwell which gives access ONLY to the upper flat. We don't know how it looks but we know it has a stair lift fitted (as the palm print was found on the rail for it). Fire safety laws in Scotland provide that any dwelling over 3 or more floors has self closing doors into all rooms off any stairway so it SHOULD have a self closing door at both the top and bottom of the stairs. It doesn't follow that both would have a yale lock, and unless you want every kid in the neighbourhood in playing on the stair lift, then you IMOO you would lock the bottom door.