As far as I can see you would beonly be responsible for the 6 months, but double check as to whether the original agreement had a clause that let them extend the tenenacy under the original terms and conditions, if it has then you could still be liable. If that is the case, contact both the person you are guarentor for and the rental compmany / person, stating that the original greement was 6 months, and therfore you are no longer liable.
Any documents / letters sent, send via recorded delievry.