Charging old fuel prices

Went to put in petrol after price had come down and being on my way to work, just took the slip and left. Several days later, went to same garage and came home and compared the prices and found that I had been charged the old price the first time. Approached garage but manager was not available and clerk explained that the information reflected on the cash slip did not actually indicate the actual price at the pump as the two do not \speak"to each other. Told sorry but that sounds crazy and that I will be taking the matter further. Was on my way out when another clerk approached me and I showed her the two cash slips. All she did was take my contact details and get the manager to contact me - needless to say I am still waiting on that call. Subsequently submitted a complaint on the official website of BP but also no response."""