There also seems to be an earlier occasion when they had a 'candy-pull' in the college kitchens, which I suspect influenced my account (judging by the reference to the thermometer belonging to the 'cook', which wouldn't have been the case if they were making fudge in their rooms -- I wonder how? Over the study fire?)

Ironically, being now old enough to have made fudge for real on my own account, I know that you don't actually need a thermometer at all to test for the right heat... and in that era they would certainly have done it the way my old recipe instructs, by boiling the mixture and testing it consecutively through the 'thread', 'pearl' and 'soft ball' stages in cold water. Fudge is actually the easiest of the boiled sweets to do, as the sugar stays at the 'soft ball' stage for quite a long period as you heat it before getting any harder.

I did question mentally whether it was canonical for Judy to refer to 'Master Jervie' at that stage in the story, but the fudge references in fact come quite late on -- after the stay at the farm -- so it's appropriate after all. Almost certainly by coincidence, I imagine, since I wasn't nearly so fussy about that sort of thing at the age of eleven!