Great composition, window in perfect position!'There is no point pressing the shutter unless you are making some caustic comment on the incongruities of life. That is what photography is all about. It is the only reason for doing it.'......Philip Jones Griffiths
Having checked the horizon on this and a few others plus having visited the area again and checked using a spirit level the only conclusion i can draw that its the natural curvature of the earth,having looked at a few shots taken from similar angles by Joe Cornish only confirms this
Great photo. I notice you are using a wide angle lens. Curvature on the picture is typical of wide angle lenses but I assume that this was taken at nearer 40mm because it is very slight. I read somewhere that at 24mm you take in over 90 degrees of view. Not sure if this applies to DSLR though.
I think I'll put a wide angle on my xmas wish list... along with the canon eos5d, the 70-300mm image stabilised lens, the ext. flash.......