Hi Johnno,
The term Watchet refers to a blue colour thought to be from the colour of the woolens worn by French sailors in the 18th Century. The Watchet flies you mentioned were originally dubbed and also had peacock herl heads. I have dressing for the Brown and Yellow Watchet as well as the Grey Watchet which date back to the early 1700s. Pritt's Light Watchet is in fact Jackson's White Spinner and not a Watchet at all, why Pritt calls it thus is not known.
I know that Pritts book is largely copied from an earlier book and fly list that have over time been forgotten. Pritt was given some of his flies by Brumfitt of Otley but is not until you find out where Brumfitt got his flies from, that all become clear.
I have the book that Pritt copied from as well as the source fly list with the flies attached

Unfortunately Pritt played around with some of the flies in his book and in some cases incorrectly named them. His book openly contradicts some of his newspaper articles about the flies as well as previous angling books. We have to remember that Pritt in all probability never dressed a fly in his life, it is known that he bought his flies from a number of famous Dales fly dressers so in fact he got his information about the flies sometimes second and third hand.