NavList:

Thanks for your detailed reply. It has helped put things in context.

I take your point that there is a difference between what is written in text books and what navigators were actually doing.

I have previously heard that there were flaws in Cotter's work but his two books I have (one pdf, one paper) are still my "goto" source rather than the AdmiraltyManual of Navigation or Nicholl's Concide Guide. I have 1943 and 1958 copies of Bowditch but  as far as I can tell "obsolete" material is  culled from each new edition meaning they do not help me with early 20th century navigation.

I have plotted two point and point/azimuth Sumners and found the two point method much easier. Maybe that is a reflection of my limited plotting equipment. Your comment that the two point version can be plotted on plain paper is very useful. All the more reason to do more two point Sumners (-;

I acknowledge that I used the term mercator loosely.

I think I have misunderstood the term latitude by double altitudes. I had thought of it as position by two posiion lines (hence two altitudes measured). Clearly that is incorrect and I need to investigate further.

I note your comment about navigation being codified during the second world war. This is probably an oversimplification but the milestones were  chronometer,  Sumner,  Mark St Hilaire and second world war?