Poor disgusting service . Changing agreements when it suits them

. The service with nimble is disgusting and very poor . I had an arrangement with nimble to settle my debt on a one time payment. I made the arrangement on the 30th of November and l was told l have until 30th of December to make a payment of the arrangement will become invalid . I made my payment on the 13th of December . When l contact them for a paid up letter , they are changing the story that my agreement was only for the 30th of November . I have the messages when l clearly asked how long l have to make a payment and l was informed until 30th December. How can l resolve this . Who do l speak to