Information on a book needed?

It came out in the early 80's, possibly in the 70's. I can't remember the title and I'll be amazed if anyone has ever heard of it.

The story is about this girl who lives with her mother and younger brother. The brother is poisoned by a sticker given to him by a creepy man. The girl (in high school) and her new boyfriend go to the boyfriend's aunts, who are witches, to find a way to save the brother. The witches perform a spell that she must drink, which opens her to her natural powers and she fights the creepy man (a wizard).

As I said, it's an old and obscure book. It might have been English (as in, from England) since I think it was set near London.