Thanks for your question. I will try to help.
Unfortunately, I am afraid that you will most likely be bound by the tenancy agreement which you have signed and executed as tenants.
If you were provided the tenancy agreement for you to sign then it would have been for you to review the tenancy agreement and ensure that the terms of the tenancy were suitable for your purposes. Ultimately, this may sound harsh but the Agents can not be held responsible for a tenancy agreement which you signed.
The Agents may have referred to a 12 or an 18 month contract as being agreed in principle but this is subject to the written tenancy agreement that would have actually been signed. Therefore, a cursory check of the tenancy agreement would have revealed a longer term and it would have been for you to decide whether or not this was suitable. If it was not suitable you could have of course re-negotiated.
It is highly unlikely that you will be able to recover any losses that you suffer from the Agents. The difficulty is that although they may have said it may be a longer term they have not strictly mis-represented the terms of the tenancy because they have provided you with the full tenancy agreement with the actual term of the tenancy agreement stated in it.
For this reason and I am very sorry that you will not be able to recover any monies from the Agents.
You can of course posture that you will sue them or perhaps make a complaint to their Association in the hope that they may perhaps reduce the rent very very slightly but I would ultimately not expect this to be acceptable, it may be worth a try though.
My goal is to provide you with a good service. If you feel you have received anything less, please reply back as I am happy to address follow-up issues specifically relating to your question.