The forfeiture of a headlease means that all sub-leases will also end. But, the position of a sub-sub-T can vary. Consider these two examples: 


a butcher has a sub-sub-lease of an over-rented shop. The head T defaults on the rent and L forfeits. This means that all sub-interests, including the butcher’s sub-sub-lease, fall away. Since the shop rent is high, the butcher is pleased – he takes a new lease elsewhere. Four months later, the head T pays off the arrears and obtains relief from forfeiture. The effect is that all sub-sub-interests are reinstated. The butcher therefore finds himself involuntarily liable for the ongoing sub-sub-lease rent! See Dende [1910]; 


a baker has a sub-sub-lease of an over-rented shop. The same default by the head T, and subsequent forfeiture, occur. The baker takes a new lease of other premises. At this instance, the head T does nothing, but the sub-T (the baker’s own L) applies for a vesting order under s146 LPA 1925 and obtains a new lease for the duration of the old sub-lease. This does not reinstate the baker’s sub-sub-lease. The baker happily remains free of it (Hammersmith & Fulham [1990]). 


The point to appreciate is that the continuing liability of the butcher/baker (ie the sub-sub-T) depends on the choices made by a superior T over which they have no control. But the outcomes are very different. Source: Herbert Smith.