Pay back the money

I made an overpayment on my account after I cancelled my account. I submitted a claim for a refund on 18 July 2017. I have followed up on numerous occasions and get told different stories I have not been paid my money back. Where do these complaints get edcalated to? Do i go to the small claims court? Is there an ombusman to complain to? I absolutely cant bear to contact the call centre again. Its such ridiculous service