Goodness, Chris, it's a very long time since I read all the "Swallows and Amazons" books.......altho I do still have a few on my bookshelves!
I think Wild Cat Island was based on an island in Coniston Water?
I remember climbing Coniston Old Man, only to have a mist come down...and we couldn't see a thing by the time we got to the top!
So, do you think the "Silent, upon a peak in Darien"" at the end of Keats' poem, is what was referred to in the Guardian clue?