Refund of agents charges

Tenancy Types and Management

Hi, I had a 1 year tenancy with a 6 month break clause, set up by Country Wide Residential Lettings, on for a Let Only service, as I manage my proprty myself.
They billed me for the full years tenancy, but said at the time, that they would refund if the tenunt desided to leave by the 6th month opt out point.
The tenunt did leave giving notice etc, both to myself and the agents, but the agency is still refusing to refund the 6 months of fees, quotting that it is a "Let Only service it provided".
Is this correct?
What can I do to recover this £468.83 difference ?

Hope you can help,
NN