Anything authorised or required by the provisions of this Act, other than subsection (2), (3) or (4) of section 23, to be done at any time by, to or with a landlord, shall, if at that time the estate of the landlord in question is subject to a mortgage and the mortgagee is in possession or a receiver is in receipt of the rents and profits, be deemed to be authorised or required to be done by, to or with the mortgagee instead of that landlord.