If a sub-mortgage by conveyance of the fee simple is subsisting immediately before the commencement of this Act, the principal mortgagee shall take the principal term created by paragraphs 1 or 2 of this Part of this Schedule (as the case may require) and the sub-mortgagee shall take a derivative term less by one day than the term so created, without impeachment of waste, subject to a provision for cesser corresponding to the right of redemption subsisting under the sub-mortgage.