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I've never had any problems at Banbury. Both the north and south ends of the station allow for decent photographs, as does the road bridge just north of the station. I find the southbound platform to give better shots, due to the curve of the line there.
I too have never had any problems at Banbury. As for the platforms, I usually stand at either end of plat 2 or 3. Both will allow for decent photo's etc.