18. Where the qualifying dwelling-house is a flat, a provision of the lease or of an agreement collateral to it is void in so far as it purports—
(a)to authorise the recovery of such a charge as is mentioned in paragraph 16A (contributions in respect of repairs, etc.) otherwise than in accordance with that paragraph and paragraph 16B (restrictions in initial period of lease); or
(b)to authorise the recovery of any charge in respect of costs incurred by the landlord—
(i)in discharging the obligations imposed by the covenant implied by paragraph 14(3) (rebuilding or reinstatement, etc.), or those obligations as modified in accordance with paragraph 14(4), or
(ii)in contributing to costs incurred by a superior landlord or other person in discharging obligations to the like effect; or
(c)to authorise the recovery of an improvement contribution otherwise than in accordance with paragraph 16C (restrictions in initial period of lease).