Oh my, what a sneeky backhanded trick! Firstly, the agency should ask you if you even want to sign a new 6 month agreement, if you don't then you should go onto periodic. This means that the original terms of the tenancy will still apply, but you will just need to give one month's notice if you decide to leave. Secondly, any charges for further contracts should be payable by the landlord.
As stated above, it's dodgy ground as there are no real regulations for letting agents; which may mean they won't renew your contract.
Have you told them you wish to go onto periodic?