Anthony, Tom, and Paul,
Thank you very much for the references, they have cleared the matter up for me. In fact, the mention of the Royal Scots was particularly of interest, since it was a friend of mine, George W. Jeffrey, an ex-Sergeant-Major of the Royal Scots, who first mentioned it to me in 1982 when we worked on his book Tactics and Grand Tactics of the Napoleonic Wars.
For many years afterwards I checked each new Napoleonic book I added to my collection trying to find an example. As I said, eventually I did find one whistle reference but have since lost track of it. Thirty-five years and one world wide web internet later, I can collect the information with one question posted here. Amazing!