Advice, please, on changing a tenancy

Tenancy Types and Management

Hello,
I have let to two couples in the same property for nearly two years. At the last renewal (Jan 2010) they opted to convert the tenancy to a rolling one-month tenancy (as allowed for in the original tenancy agreement) to give themselves extra flexibility. In March of this year, we put the property on the market and kept the tenants fully informed.
One couple have now given notice to quit and, with the agreement of all, I have served a Section 21 to regain possession on 11 September 2010.
However, one couple have now asked that they stay on, with a new contract in their names alone, despite the fact that the wife's mother has been staying in the house for several weeks and intends to stay on as long as they do.
I want to make out a new contract in all three names but, because they are trying to be rehoused by the Council, the couple want only their names on any contract, and no mention of the mother. How can I protect myself and ensure that the mother can be made to leave at the same time as the others - is the only way to include her name in the contract?
As it is effectively a new contract does it have to be for 6 months or, as the tenants have lived there previously and continuously, can we continue on a rolling one-month basis? (As I am selling the property I don't want to commit to a six-month agreement at this stage.)
Any help would be gratefully received, as we need to resolve the legal situation with them in the next couple of days.