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Absolutely, I agree with you both. The sensible thing would be for my sister to cease to contribute to a joint account. I believe the partner cannot be trusted, and I have been telling my sister this for some time now, so I am hoping that finding out about the "hidden" direct debit will be the catalyst needed to make her do something about the situation.
Thanks for the opinions on the likely position the bank will take. I thought, too, that they would refuse to give any information.
Many thanks for the answers.