Check the contract first and foremost and then speak to the agent.My understanding from when we have rented ours, under an Assured Shorthold tenancy,is that it is for a minimum period of 6 months but when extended the lease itself is not for a finite period of time but that contract may only be valid for a year (ie. the rent and terms are fixed for that period and can only be reviewed at the end of it) so the tenancy can still be broken by notice periods specified in the contract in the meantime. Could well be that the Landlord has misunsderstood the extension to the contract if the agent has not made it clear. CAB should be able to read it through with you to clarify your position.
Good luck