The Chalet School and the Island (by Elinor M Brent-Dyer)

In my current meandering re-read of Elinor M Brent-Dyer’s long ‘Chalet School’ series, I reached the one which was 21st in the original series, ‘The Chalet School and the Island’.  I last read this in 2011 and had only the vaguest memories of what it involved. 
I previously had an Armada paperback edition of this book which was falling to pieces, so was pleased to be given an old (but well-kept) hardback by a relative who was getting rid of some inherited books a few years ago. However, accord...