Thanks for your reply.
The key issue is whether the new tenancy is binding upon you. If you signed the tenancy agreement and the agent/landlord signed/date it then it will be legally binding from the date your previous tenancy finishes I'm afraid.
If the new tenancy agreement was not signed/completed by the landlord then you tenancy would have become a stautory periodic tenancy if you stayed in the property after the expiry of the previous tenancy (which I believe you did. This means you would have had to have give the landlord one months notice in writing with such notice to expire at the end of a rent period (ie. which I assume was 6th of the month), so the notice you have actually served is not an effective notice.
No doubt the agent will say the new tenancy has been completed. Ask for a copy of it. If it has then you are bound to pay the rent until you can terminate (see if there is a "break" clause permitting early termination) or until a replacement tenancy is found. Landlord's are under a duty not to refuse a unreasonably replacement tenant.
If the new tenancy agreement was completed then you should either focus on finding a replacement tenant to produce to the agent (ie. http://www.gumtree.co.uk/ perhaps offering to pay the new tenant's pre-contract agency fees) or offering the agent some money in order to get them to surrender the new tenancy (ie. mutual agreement to terminate the tenancy - which you would need in writing) in the hope that they will take the opportunity to take your money and get another tenant in there in order to effectively earn double rent for a short period of time.
If the new tenancy is binding and you don't pay then they will probably sue you for the debt and would likely receive judgement on the amount of rent they would have earned in rent until they could have resonably found a replacement tenant (usually two months or so)
Sorry it could not be better news.
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