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Geoff Hitchcox, thank you very much for tracking down both the British Sundial Society bulletin and the paper by Fred Sawyer deriving a formula for the latitude error - they are both very interesting.  The formula allowed me to check satisfactorily the points on my graph of the sundial time error (derived by iteration), and as you point out, very much quicker!

As in your 13^th June post, I had started to think about the sundial errors that we believe we can see in the photos, and the fact they appear to be somewhat greater than explained by the wrong gnomon angle. Like you I wonder if there might be an additional factor involved. Perhaps there might be a clue in the time that the sundial indicates at solar noon, with the error arising from the gnomon angle known to be zero then.

Regards,

Adrian F