Right you are now occupying the tenancy under an assured shorthold tenancy agreement which means that your tenancy continues from one rent period to the next but otherwise on the same terms as your previous tenancy.
You are required to serve one months (if rent is paid monthly) notice under a statutory periodic tenancy agreement, regardless that the AST referred to a two month notice period. This notice must expire on the final day of a rent period in order to be effective. A valid notice that you could serve therefore must expire on 27th of the month which you wish to terminate - the notice you have served is therefore invalid.
In fact the agent could simply wait until the 28 August and when you tried to vacate state that you have not served a valid notice under the SPT and require you to serve another notice. He might be good to his word to accept your earlier notice as being effective on 28th September, he might not - I personally would not take the risk.
My advice would be to serve a further notice upon the agent now in writing referring to the statutory periodic tenancy and giving him one months notice with that notice to expire on 27th September, swallow the extra money and write it off to bad experience.
If you don't think he would sue you for the money if you vacated earlier then you could take the risk, but it's very easy to sue for rent arrears and 95% of agents would sue for that amount.
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Edited by Thomas on 8/21/2010 at 9:01 AM EST