It appears that TextureCorner is being ignored by the font structure.
In my font sheet, I had an 18 pixel gap on the left hand side of the sheet which meant all letters were slightly offset.
TextureCorner = (18, 0, 0)
...made no difference so I simply cropped off the left 18 pixels in gimp and all was well.
So this is easily solved, but just reporting this as TextureCorner is listed as available at: