Freeholder playing dirty tricks

I have a dispute around ground rent with the property management company and desperately need some advice. 

I am the owner of flat and as per the lease agreement there is a £200 ground rent charge that I am liable to pay annually. As per the agreed procedure, they send me a letter demanding ground rent, after which I make the payment.

This year, the first letter I received from the management company was on the 14/02/15 which had the ground rent charge along with late payment charge of £35. I immediately called them to enquire about this late payment charge, as this was the very first letter I’d received from them. According to their agent they had sent an initial letter on the 17/11/14, which I never received. On the same call I offered to pay them the ground rent of £200 excluding the late payment charge. They declined to accept the payment on the grounds that I have to pay the entire amount including the late payment charge as well. When i challenged the late payment charge, I was asked to write an email explaining the situation. I did exactly that, but never had a response back from them. I have emails with date and time stamp to show that the email were send to the management company as requested.

The next I heard in this matter was another letter, this time from a property debt collection company with an additional fee of £235 on top of the ground rent, for total charges of £435!!!

I have now written letter several letters to the company requesting to explain the reason for these charges when I had offered to pay in the first instance. I have explained that I wrote an email to them, as per their request, when their agent declined my previous offer but never received any clear response from them.

Can anybody advise what I should do next please?

Regards

Arun Aiyer