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Many thanks to those who answered this question.  I’ll answer with a single post if you don’t mind. 

Tony Oz, Donny Maracle & Mike Mayer thanks for the links.

Wolfgang Hasper. My son is a CAD/CAM designer.  I’ll see if I can persuade him to put something together.  

Robin Stuart.  Thanks for the information re small and great circles mapping to circles on the plane.  That explains why the Almucantars are also circles.  All I have to do is calculate the position of their centres and their radii (he said as if that was some trivia task).  

Geoffrey Kolbe.  Believe it or nor there are two copies of ‘Practical Astronomy’ on Abe Books for sale in Lincoln, but in the current situation they might as well be a hundred miles away.  One is £5.74 and the other is £9.13.  I’ll ring up tomorrow and check on the difference in their condition.  I think we launched our paper darts from the 11^th floor.  The high-speed lifts would have been great for solving those ‘Man in lift’ science problems.  I can’t think why I never took a set of bathroom scales in with me, probably because the pre-digital ones were quite expensive.

DaveP