Unauthorised debits

I had unauthorised debits that went through my account and raised a query on them. They were blocked in Dec 2017 and assure me that they have cancelled. In April another debit went through when I ask they said it's a mistake and they are quick to exercise a refund. The fact that funds are refunded doesn't take them off the book. I requested a mandate that authorised the initial debit and they are unable to produce that....i therefore will like take them on lawsuit.