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Frank you said :

Call him up again, and ask if my speculation was right that this puzzle was inspired by the optical analogy and Fermat's Principle of Least Time. :) As I say, I have never seen this puzzle before or anything similar but it jumped right out at me that it was similar to an optics problem.

The puzzle was set by a Lecturer in Astro Statistics.  You can check or ask him yourself; I might have used up all my credit with IC.  One of the advantages and disadvantages of Google is you just put in the name and a university, and up pops the academic.  I was going to email him, but my new Microsoft Edge went off in a funny direction when tried the email link, so I dialled the number instead, and he picked up the phone.  I'll send you a link privately.  It seemed odd that someone used to dealing in light years came up with a problem in yards.  DaveP