3Where—
(a)under the terms of the tenancy, if the dwelling-house was let at the relevant date, or,
(b)under the terms of the last subsisting tenancy, if the dwelling-house had been let before the relevant date but was not let at that date,
the landlord is or was responsible in part only for the repair of the dwelling-house, the two foregoing paragraphs shall have effect respectively with the substitution for the values therein mentioned of those values proportionately reduced.