This section applies in any case where—
the premises comprised in the sub-tenancy or any part of the premises, but
Where there is served on the landlord a written application by the tenant for approval or a copy of a written application to the tenant by the sub-tenant for consent to a transaction to which the covenant relates the landlord owes a duty to the sub-tenant within a reasonable time—
Giving approval subject to any condition that is not a reasonable condition does not satisfy the duty under subsection (2)(a) above.
For the purposes of this section it is reasonable for the landlord not to give approval only in a case where, if he withheld approval and the tenant gave his consent, the tenant would be in breach of covenant.
It is for a landlord who owed any duty under subsection (2) above—
and, if the question arises whether he served notice under that subsection within a reasonable time, to show that he did.