Breach of a debit order mandate

I have experience terrible customer service with their call centre when lodging a complaint with double debit orders on my account. They have breached their debit order mandate by debiting me twice amount of the mandate and when trying to reverse they blocked it, I even call my bank, and they were unable to reverse it. I call them to cancel my debit order, and they said I don't have the account with them while debiting my account. I'm frustrated now because they're not responding to my calls. I advised my partner to contact them personally because I was paying her account with them through debit order. I'm intending to take legal route if this is not resolved with immediate effect.