The first children’s book written by the Godmother of British fantasy, Diana Wynne Jones.
OWN BACK LIMITED REVENGE ARRANGED, PRICE ACCORDING TO TASK

Frank and Jess set up Own Back Limited because they need the money - so why not offer to do the bad things people don’t dare to do themselves? Their first customer is the neighbourhood bully. After his tooth is knocked out by Vernon Wilkins, he demands a tooth from Wilkins in revenge.

But the local witch, Biddy Iremonger, sees Own Back Limited as a threat to her own evil schemes. And when Biddy is angry, there’s no end to the trouble she’ll cause …