It is the tenants' choice as to whether they want a new fixed term agreement, not the landlord's choice. In my opinion, for the landlord it is really irrelevant as the tenants may still get out of a fixed term tenancy whereas the landlord cannot (unless there are specific clauses in the lease which would allow him to do so.
I presume the last contract was a fixed term for 1 year. Therefore, the tenants have acquired their Part 4 rights (after 6 months in occupation) which include not having to sign a new fixed term lease. The initial agreement just continues as a Part 4 tenancy. Any special conditions that were in the original contract continue (such as not pets allowed).
If the tenants want more security of tenure they may request another fixed term agreement. You could just use the original contract and edit it accordingly. However, it may be copyright.
You can download a lease agreement from several websites, which may or may not include parts of the new Acts now in place.