Fiona, a tenancy extension agreement can be a simple one page document (no need to do a fresh multiple page tenancy agreement, especially since both you and the landlord have agreed to continue on the same terms and rental amount) making reference to the previous agreement, stating that parties have agreed to extend the term of the tenancy for xx period, and which incorporates the previous agreement (and its terms) by reference.
The suggested wording incorporating the terms of the previous agreement could be as follows: "All terms and conditions of the 20xx Agreement except as herein otherwise amended are hereby incorporated by reference and shall continue to apply with full force and effect during the term of this Agreement."
Hope this is useful, especially if you try to get out of the outrageous demand for commission from the agent who did absolutely nothing but simply reprinted the same original tenancy agreement with the dates changed and is saying that you need to pay him/her for the "work" done in preparing the agreement. Good luck!