Since you mention in your comment to the other answer that your contract is with the agent and not the landlord, it sounds like one of two things may be happening :
- The agent's collection department is not aware of the landlord's withdrawal from his contract with them
- The landlord's effort to terminate the contract with the agency has not been accepted by the agency
On one hand you have a written document (the contract with the agency), on the other it appears all you have is something someone has said. The contract will carry more weight in law.
It may be that the landlord has effectively withdrawn from his arrangement with the agency, in which case your contract with them has been "frustrated", as they are no longer able to supply their part of the agreement (the room / flat / house).
It would be worth responding (also in writing) to the agent with something like :
"[Landlord's name] has advised me that your agency will no longer be representing him after 1 September. Can you clarify what I will receive for any payment I make to cover the period 1 - 15 September, and to whom this payment should be made?"
It is important to make sure you don't accept that you "have refused to pay". You have delayed payment pending clarification.
This should nudge the agency into confirming that they are legally able to offer accommodation at that address in consideration of your payment to them - in other words it should clear up any confusion about whether they have accepted the withdrawal of the landlord from the contract they signed with him. If the answer to that is that they are unable to offer the room / flat / house, your contract with them has been frustrated and you will not be liable for "rent" (I've used quotes there, as it's not rent if there's no accommodation available. Worth also noting that I'm using "accommodation" from estate agents', rather than from lawyers', terminology in this paragraph.) It will also give you evidence that you're not refusing to pay, just asking for clarification.
If that doesn't provide a satisfactory (to you) answer, it sounds like you might be caught in a dispute between the landlord and the agency. That's when taking professional legal advice becomes the safest way to proceed, though since we're in the UK the local Citizens' Advice Bureau would be a good place to start.