The landlord cannot recover possession from a licensee of an absent tenant unless the tenancy has been brought to an end.
If the tenant no longer occupies the property, the tenancy cannot be assured and becomes a common law tenancy, which can be terminated by a notice to quit (NTQ). Similarly, a secure tenancy will lose its secure status.
Tenant is dead
Assuming that there is no successor (spouse plus 12 months' residence if periodic assured tenancy, family member plus 12 months' residence if secure) the tenancy will continue to be held by the tenant's estate. Note that there can be no succession to a fixed-term assured tenancy, which will devolve according to the tenant's will or the rules of intestacy.
In theory, it is possible for any tenancy, as real property, to be left to another in the tenant's will, or to devolve according to the rules of intestacy. In order for this to happen there has to be either a grant of probate (will) or the grant of letters of administration (intestacy). Until this happens, the tenancy is held by the tenant's named executors (if there is a will) or by the Public Trustee (intestacy) on behalf of the tenant's estate, and can be terminated by service of a NTQ. The NTQ should be served on the personal representatives of the estate of the deceased tenants at the property and on the Public Trustee.
Tenant has moved to new accommodation
The tenant will lose its statutory protection and can be terminated by service of a NTQ by the landlord or by the tenant (if the tenancy is periodic), or by surrender by the tenant. Most tenancies deem service on the property as valid service, but do check. If the tenancy is not clear, it is best to also serve a copy on the tenant's new address, if known.
If the tenancy has been properly ended in this way, then again any licensee is a trespasser as far as the landlord is concerned. Proceedings may be commenced against the occupier if their identity is known or against 'persons unknown' if it is not.
Tenant has disappeared
The landlord may not know of the tenant's whereabouts or whether he or she has permanently left the premises. The landlord cannot therefore be sure that the tenancy has lost its statutory protection. Serve NTQ and 'without prejudice' s8 notice citing breach of tenancy agreement (for failure to reside as only or principal home and/or for unlawful subletting).