British YA novel from '80s - girl becomes a witch to save her little brother

I loved this book as a child and for the life of me cannot remember the name or author. The plot: a girl has a little brother. A creepy older man puts a stamp on the brother's hand that is a tiny picture of himself, and the little boy begins to get sick as the man is energetically "feeding" on him. She knows a boy at school whose mother and grandmother are witches (there's a whole interesting bit in there when she goes to his house for the first time, where they give her slices of tomato on bread with salt,and tell her that now she's eaten their salt, she'll return to visit. I ate open-faced salted tomato sandwiches for *years* because of this!). The boy and his family "initiate" her into witchcraft, and she puts her own stamp on the old man who is feeding off of her brother, and is able to reverse the curse and save her little brother. I have a vague memory of something in the book about a cup, a coin, a sword, and a bell, maybe? That may or may not have been in the title as well? Any help would be swell! Thank you!

This is The changeover by Margaret Mahy. It's actually from New Zealand, but everything else matches your description.

It sounds great! I put it on my wishlist!

Thank you so much! That's the book!

Glad to have helped! Mahy was a terrific author; check out The tricksters and Catalogue of the universe as well.