Many thanks for your help. I'll try this thanks again

Morning Everyone, First of all many thanks for taking the time to read and help me with my problem. A few years ago I had a overdraft with HSBC which I defaulted on. They instructed Moorecroft to collect the debt which I paid back of a 4 year period. I finished paying this late April. I then received a letter from Moorecroft saying the account had been settled and I had actually over paid by £40.00. Moorecroft told me that I would need to contact HSBC to arrange for this money to be returned. I called HSBC who wouldn't speak to me as my account number and telephone banking details are no longer valid. I went into my local branch who advised me to call Moorecroft as they have nothing more to do with me. Moorecroft said the same. I emailed both HSBC and Moorecroft on the 17th June asking who was responsible for paying me back. I only received a response from Moorecroft saying it was HSBC's responsibility. To this date I have still not received any response from HSBC. What would be my next course of action? I would really like to start charging them interest for not paying me back like they did me. I know it's not large amount of money but its the principle that they hassled I'd like them to know how it feels Any help would be very helpful.