Special levy instalments added though already paid

I received a letter regarding a special levy late due to the Post Office problems - I had to reply before a certain date that I would pay the special levy at once or make use of an instalment offer. I could not inform them that I wanted to pay at once, as the date was already due and I received the letter a month late. I phoned Trafalgar immediately, explained about the letter I received a month late and informed them that I will pay the levy immediately - WHICH I DID. It was only 10 days later than the date we were supposed to inform them of once-off or instalment. <br> HOWEVER - due to Post Office, I received only today (19 October) the Trafalgar statements for June, August, September and October - still no statement for July, and saw that they added the instalment special levy to my account each month, though I have already paid it!! Of course there is a lot of interest involved. To crown it all, there is also an amount added for \other necessary expenses"! How can they charge interest on an amount that is already paid? They are quick to collect their management fees"