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Many thanks for the advice.
The lens is a Nikkor 50mm f/1.8d AI-S Prime lens.
I shall have a play around and crop the mentioned point in the first picture, I can see where the distraction comes in from that slightly. I shall do the same in the third picture as you mentioned and see the outcome.
As for the grid, I don't think this is possible on a D3100, however I used a grid to align these pictures at certain point, for example I used the right post of the building in the first picture to determine a vertical plane.
Once again, many thanks for the advice!