Leases registerable under this Act, and valid and binding as in a question with the granters thereof, which shall have been duly F2registered or recorded, as herein provided, . . . F3 shall, by virtue of such registration, be effectual against any singular successor in the lands and heritages thereby let, whose F4title is completed after the date of such registration: Provided always, that, except for the purposes of F5, and subject to section 20C of, this Act, it shall not be necessary to record any such lease as aforesaid, but that all such leases which would, under the existing law prior to the passing of this Act, have been valid and effectual against any such singular successor as aforesaid, shall, though not recorded, be valid and effectual against such singular successor, as well as against the granters of the said leases.