The curve starts at the 4th arch from the Brook Street end, this within the covered station area. It is not really possible to see this in a photograh as the change of direction is on the line of a down pipe.
I suspect that the line follows a curve in the existing road given that these two platforms were added at a later date.
The roof line at the opposite end is different and does not meet the side wall of the station in quite the same way.
Link below is to an opposite end view of the wall.
https://www.facebook.com/ilkleychat/photos/pb.518436761577943.-2207520000.1460206722./911728535582095/?type=3&theater
regards
James