Transfer of tenancy to someone else

Tenancy Types and Management

We have a situation where the woman (single tenant) on our RLA tenancy agreement has moved out but her boyfriend wants to take over the tenancy. Clause C9 obliges the tenant not to transfer the tenancy to someone else without our permission, in writing (which we will not unreasonably withhold). The initial AST has tripped over to a periodic. Is there a standard RLA form whereby we can transfer the tenancy, or would a letter signed by us and new tenant saying all references to 'x' are now replaced by 'y' with effect from the date of this letter suffice? We will have to deal with deposit separately as that was in initial tenant's name.