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As the fixed term of your Tenancy has ended, you have what is called a "periodic tenancy" and the terms of the existing Agreement remain in place until eiither party gives Notice to end it.
A Tenant can end it by giving 1 month's Notice, such Notice to expire on the day before the day in the month on which the rent is paid.
Therefore, on the basis that you pay rent on the 1st of each month, to have ended the Agreement on 30th November, you needed to have served Notice on or before 30th October.
As you did not give Notice until 7th November, the Agents are right in that legally, your request to end the agreement on 30th Nov/1st December is invalid, and they are therefore entitled to the rent for December also.
I am sorry this is not the answer you were hoping for, but it sets out the legal position. There is no point in you pursuing or arguing matters, as the law is clear on the issue of Notices.