It shall be lawful for any heir of entail, being of full age, and in possession of an entailed estate in Scotland . . . F1 to acquire such estate, in whole or in part, in fee simple, by applying to the Court of Session for authority to execute and executing, and recording in the register of tailzies, under the authority of the court, an instrument of disentail in the form and manner herein-after provided: Provided always, that such heir of entail in possession shall be the only heir of entail in existence for the time, . . . F1 or otherwise shall have obtained the consents of the whole heirs of entail, if there be less than three in being at the date of such consents and at the date of presenting such application, or otherwise shall have obtained the consents of the three nearest heirs who at the said dates are for the time entitled to succeed to such estate in their order successively immediately after such heir in possession, or otherwise shall have obtained the consents of the heir apparent under the entail and of the heir or heirs, in number not less than two, including such heir apparent who in order successively would be heir apparent: Provided also, that the nearest heir of entail for the time entitled to succeed to such estate immediately after such heir in possession, where any such other heir exists, shall be of the age of F2eighteen years complete, and not subject to any legal incapacity.