I've just checked the fret with a straight edge and it does rock (and not in the good sense) over the 11th. I've also been horrified to discover I can get a 0.15 mm (0.006 inch) feeler guage between the fretboard and the flange(?) of the fret at its centre point.
There only seems to be a gap here though,the fret is square with the fretboard at both ends. Its almost like a fret with a slightly larger radius has been put in, but its difficult to compare with the other frets (I've got 4 red lines on my face to prove it

). Can this happen? If so how can I prove it? And why doesn't it affect the A string
Sorry for all the questions, and thanks for all your help in getting this far.