I take it that you have inherited a Flat, which is held under a Lease.
If so, whenever ownership changes, the Purchaser or new owner is under a duty to serve formal Notice of Transfer on the Managing Agents. The Lease will explain in full what is required- for example, there may be a Deed of Covenant that you may need to sign, which is basically a document confirming that you will pay the service charge/ the Agents may also need to deal with issuing you with a Share Certificate if you are to become a member of the Management Company or own a share in the Freehold.
I'm afraid all these different issues attracts fees- although all Managing Agents fees are astronomical, £450 is the usual fee if all of the above issues need to be dealt with.
I would be happy to clarify matters further if you could confirm what exactly the M Agents have asked be dealt with.