Read a children’s book in the 1980s; it may be older. I remember two or three children, boys and girls (siblings?), maybe in London, England, who encounter coven of witches. I think there is at least one chapter involving the kids shrinking in size. At least one scene of Broomstick flight.
Is it Ruth Chew’s “No Such Thing as a Witch?”
Possibly Dorrie and the Blue Witch by Patricia Coombs?
I think that book you are looking for is called “The witches brew” by Ruth Chew
Sounds like John Masefield’s The Midnight Folk (http://en.wikipedia.org/wiki/The_Midnight_Folk)
Maybe Mary Stewart’s The Little Broomstick? Or Mary Norton’s Bed-Knob and Broomstick? Or (in the US) Jane Little’s Sneaker Hill?