Had an unauthorised direct debit from my account

Last week I received a msg from my bank saying an amount had been deducted through direct debit from Transunion regarding sage pay. I am not registered with Transunion nor have I even heard of them before. Made numerous calls which have been costing me alot and emails and absolutely no response from them. Extremely frustrating as I cannot get the answers I need as to how did they get my banking details and why was this amount deducted!