Additional tenant sharing a room

Tenancy Types and Management

I have a 4-bed house with 4 tenants on a joint 12m AST - ending July 2010. One of the tenants has asked if she can have a friend sharing her room with her until February (ie 6 months), in order to reduce her costs. The other tenants are cool about it too, but will probably want to split the utility costs 5-ways. I don't have a problem with another sharer, but would like advice as to which course of action to take. Either, giving agreement by letter that the tenant can share her room with named person (anything else in the letter?). Or, have her as an additional tenant, take references, add her to the AST etc.

A fifth person in the house would take me over the threshold for licensing as an HMO (it's a 3-fl house). The license would be needed even if the 5th person is a sub-let, and will cost me the license fee of £975 for 5-years. I have not checked my insurance to see if a 5th resident will increase the premium.

I am prepared to let the 5th person in the house if I could recoup some of the addtional costs - and half the license fee seems reasonable as it may be useful to have the license for future. Would it be OK to charge her a different figure than the others and it still be a joint AST? or do I have to add her and amend the amount I get from the person she is sharing with? - or can I have her as a sub-tenant and charge her a license fee?

Looking forward to hearing from you.

Ruth