Their FAQs on the matter is not giving any magic formula unfortunately. The only thing you can do is try to resolve the cause of that negative entry as soon as possible and keep the rest of your financial related affairs in order. This mainly means paying all your bills and credits in time, and never use your overdraft possibilities if possible. Negative records are impacting your score up to 3 years after you resolved your unpaid bills case and it can be kept as a "FYI side note" for up to 6 years overall. See a detailed table hereafter.