SUPERGRAN IS MAGIC by Forrest Wilson.
Super Gran wasn't all that bothered about Mr Black's new invention : a small black box that could hypnotise people and animals. But then a rotten stage magician called Mystico thought of the perfect way to make his act more exciting : he'd set a hypnotised Super Gran to work for him. So suddenly Super Gran had to call on all her Super-powers!
Puffin paperback in very good condition