80's and 90's childrens book?

searching for title/author of book I read to son, middle 90's. similar to Mary Poppins. A woman started looking after 3 children-only remember the start-she joined the library, then realised she had no money, and messed around with cashpoint,using library card, and getting hundreds of pounds, free-the little... show more searching for title/author of book I read to son, middle 90's. similar to Mary Poppins. A woman started looking after 3 children-only remember the start-she joined the library, then realised she had no money, and messed around with cashpoint,using library card, and getting hundreds of pounds, free-the little girl was scared all the way home, expecting Police to arrest them all-the next day woman messed with traffic lights,(The men on crossing sign did dance-so prob story British) but can't rmember any of rest of the book except for (poss false memory)Eigg was mentioned-boy thought it was Egg-she was banquished there,so she was possibly a witch-but at the time I thought it was planet, not island.It was a small pb, the cover was her and kids I think-youngest in pushchair. Probably no replies-but worth a try. Thanks in advance
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