803. Lawful subtenancy.

A subletting is lawful if it is not contrary to the terms of the mesne tenancy1. It is unlawful if it involves a breach of covenant, even if there is no right of forfeiture2. The question of lawfulness is to be determined as at the date of determination of the mesne tenancy3 unless the mesne tenant is still a statutory tenant, in which case the material date is that of the commencement of possession proceedings4. If before the material date the head landlord has waived the breach, the subtenant is protected against him5. A waiver only operates so as