Advice re named occupier

Other

In September last year our managing agents arranged a new tenancy with a young couple, one of whom as it turned out just before the Lease was signed was only 17. In order to accommodate this, an additional clause was added permitting her to reside in the property as a named occupier. A rent guarantor is also in place. She has now just turned 18 and the Agents are in the process of completing a new credit check and issuing a new Joint Tenancy Lease free of charge.
My question is: is it necessary to create a new Lease (the term ends on 6th September 2008) as I understand this would mean we would have to register the deposit again at further cost (we are members of the insurance scheme promoted by the RLA), and a response in the RLA's member's forum I have just seen seems to suggest that she could remain as a named occupier and therefore would not have any rights to stay in the property as she was not a party to the original Lease. Your advice would be much appreciated.