When you rent out a tenancy to a sublease tenant, there are the concept of reasonable rent, which controls how much you can charge in rent. The rent paid by the primarily tenant can normally be taken as a reasonable rent to charge when the apartment is rented as sublet due to the fact that the rent has often been negotiated between the original landlord and the Tenant Association.
Should the rent that has been paid by a sublease tenant be considered unreasonable, the landlord (in this case the primarily tenant) may be required to repay previous rents and forced to reduce the rent for the continued lease. If the apartment is rented fully furnished, you can usually put on about 10-15% of the rent according to the Rent Tribunal. Are there other costs that the landlord (primarily tenant) pay, such as parking, one may also get to add that cost on the rent.