Interest charged on settled account

I settled the full balance owing on my avios credit card account (as it appeared on my online banking just before making a payment) on the 25th of March 2015 and sent a mail on the same date requesting my account to be closed. <br> <br> To my shock and surprise, I received a statement on the 29th of March 2015 and a monthly service fee and interest have been charged to my account. How can interest be levied on a settled account?<br> <br> I want the charges reversed and my account closed with immediate effect.