An instrument of revocation must be executed—
(a) by—
(i) the principal signing the instrument; or
(ii) a person, who is eligible to do so under section 47, signing the instrument in the presence of and at the direction of the principal; and
(b) by 2 persons who—
(i) are present and who witness the principal or person signing the instrument; and
(ii) sign and date the instrument in the presence of the principal and in the presence of each other; and
(iii) certify in writing in the instrument in the manner required by section 49.