Query on 'adding new tenant onto established Periodic Tenancy contract.

Tax Planning and Financial

Situation is that 2 long-term STUDENT tenants in a 3 bedded shared house were on an AST that expired and became Periodic in March 07.This suited everyone as tenants were unsure if they could recruit another '3rd' to help with the rent burden outside the usual 'student lettings' season.

This has now been achieved.The 3rd tenant will take up the vacancy next week.
My question is;
'do I have the option' to incorporate this 3rd person into the well established SPT that presently names the 2 existing encumbants?

Or, must I alternatively sign all three up into a new AST [two months alpha/ omega rent paid up front]? Separate tenancy contracts are innapropriatly anonymous and reviled by HMO Landlord License team.

In light of the revolving front door of a student house where occupancy relates to academic terms that span calenders and sandwich courses - flexible SPT's have served all parties well. Lodger/License has been tried too using the Lead Tenant device for short-stay Erasmus students.

I have not joined a TDS due to precautionary profiling of tenants and the room turnover necessary for some Uni courses.This situation would make deposit handling/tracking/returning nightmarish.

It is in no ones interest that I do so now as I have never experienced loss from theft,unpaid rent or from deliberate damage. Up front Contracts give clear definitions of theft and criminal damage,promise my zero tolerance to those and explain subsequent consequences.
The status quo of SPT appeals...BUT is it an option ?