Do I have to pay costs for ground rent demands?

I bought a house in 2008 which was a leasehold property. Shortly after I bought it I let it out to a tenant. A few weeks ago my tenant gave me a letter which was addressed to me from the freeholder demanding chief ground rent. The letter stated I owed ground rent dating back to 2008 which is correct, but also added �70 costs as they were taking recovery action. I had never received a letter before this as they were presumably going to my tenants at the property address. I have written and told them this but they still insist I am liable for the costs as well as the rent. Is this correct, as it is not my fault they have incurred these costs? I have also sent payment for the rent but they sent the cheque back.