Barclaycard informal arrangement

[Commonsense]

Hi in September I rang Barclaycard to say I was having difficulty meeting my credit card repayment and I didn’t expect this to change. They were very understanding and we agreed an informal arrangement whereby I would pay a lower amount. By October I was back in a position to pay the usual amount and I paid what was owing from September’s original invoice. I have since received a letter saying I have defaulted and a mark has been put on my credit file. I wasn’t sent a letter a fortnight earlier, as some state they had, to alert me and Barclaycard say you should have rung us to say you were making payments! Any advice would be appreciated. Thanks for taking the time.