21. (1) Before obtaining the consent of a person for the purposes of section 19, the Commissioner or an inspector shall inform the person that he may refuse to give his consent.

(2) Where the Commissioner or an inspector obtains the consent of a person for the purposes of section 19, he shall ask the person to sign a written acknowledgement—

(a) of the fact that he has been informed that he may refuse to give his consent;

(b) of the fact that he has voluntarily given his consent; and

(c) of the date on which, and the time at which, he gave his consent.

(3) An entry by the Commissioner or an inspector by virtue of the consent of a person is not lawful unless the person voluntarily consented to the entry.

(4) Where it is material, in any proceedings, for a court to be satisfied of the voluntary consent of a person for the purposes of section 19 and an acknowledgment, in accordance with sub-section (2), signed by the person is not produced in evidence, the court shall assume, unless the contrary is proved, that the person did not voluntarily given such a consent.