A problem for the Chalet School (by Elinor M Brent-Dyer)

(Amazon UK link) It’s nearly ten years since I read ‘A problem for the Chalet School’, 36th in Elinor M Brent-Dyer's original series. It's the first time in several decades that I read it in an unabridged edition. I previously had an Armada paperback which was starting to fall to pieces, so I was pleased to be able to acquire a ‘Girls gone by’ version, with the full original text. Apparently it was quite seriously abridged, with an entire chapter missing in the Armada paperback.
I vaguely recalled...