I applied for the flex account for the 5% cashback for supermarket shopping. As they cant offset, If I use the account can they write to a CRA for the old Credit Card. And can I ignore payment in full due by 23rd?

Hi All, Back in the early 2000s I got in a lot of debt. Most of this debt has been paid off through a debt management company or written off. Around 2015 all the debts vanished from my credit files due to being over 6 years old. I applied for a Nationwide flex account which I was approved for. I logged in and was shocked to find my old credit card on there with a balance owing and payment due by middle of the month. My questions are this do I need to pay this? Should I use the flex account? can they start writing to CRAs again? Nationwide have not contacted me for a number of years since around 2010 and I thought this card was part of my repayment plan. Any advice on what to do here? Thanks for all your help again. I received a lot of help from this forum back in the day.