"

 Enid Blyton's Five Go Down to the Sea 

I have just read this book.  I read the 1953 version, the earliest I could find on amazon.  I could find no evidence that anyone speaks Cornish in this book.  The nearest character is Mr Penruthlan.  He says things such as ""Oooh-ock"" (page 180), and the chidren find him difficult to understand.  However, it is explained that he talks like this because he doesn't have his teeth in.  When he has his teeth he is perfectly comprehensible in English.  I wonder where the idea that some characters speak Cornish in this book."