This is the response from the Estate Agent:
"You are entitled under the terms of the AST which is now on a periodic basis to give the landlord 1 months’ notice of you wish to end the agreement and leave. The fact that a Section 21 was served does not absolve you or the requirement to provide a month’s notice though it does mean the tenancy will end before the date that the landlord intended to seek possession. I know of no case or precedent that means that because a Section 21 has been served that you can choose to end the tenancy whenever you feel like without the months’ notice required.
Under the terms of that agreement rent is due up until the end of the last month. Only the landlord is able to say the he accepts an earlier end date requested by a tenant that may mean pro rate rent is acceptable."
No thanks I just wanted to do what is right.