My younger brother, when was about eight, in the late 1980s, bought a book from a school book order. It was probably 50 to 100 pages long, divided into chapters, with a few black and white sketch illustrations per chapter.
The plot entailed some children (a boy and a girl, I think), who meet a silly sort of wizard, whose wand is in the form of an umbrella. The umbrella is quite temperamental. It doesn't work unless the wielder is polite to it, and it doesn't work at all unless it's raining (or maybe not raining).
The kids have some adventures, including making friends with the Loch Ness monster and accidentally getting him caught and put in a zoo, from which he needs to be rescued.
I would like to find this again. What was it?