Data Protection

My mother received a letter in the post apparently from a bank, the usual blah except part of the boiler plate text started "As you know" - well no she doesn't. She doesn't use this bank, she doesn't have her name in the phone book, or the mobile register, or on the electoral roll; and, as far as I can tell, this bank isn't affiliated with anyone she does business with. So how did they get her details and why her and not my father?