Right, firstly look at your contract and see if there is break clause, some Landlords do include them in fixed term contracts. Then talk to your Landlord, the tenancy is for a fixed period, but your Landlord does have a choice in letting you end the tenancy early with no penalties even if there is no break clause. My Landlord has just let me out 7 months early, I helped him find a new tenant etc everyone is now very happy and on good terms.
If the Landlord is unwilling to do this (and he is completely within in rights to say no) you are liable for the rent for the full term � however (in the England) the landlord can no just sit back and let the tenancy runs its course he has to take reasonable steps to find another tenant.
Have a look at the Shelter website, they give excellent advice.
http://scotland.shelter.org.uk/getadvice/advic e_topics/renting_rights/renting_from_a_private _landlord