20. A person who has not obtained the requisite qualification to carry out work referred to in—
(a)regulation 15(1), (3) or (6);
(b)regulation 16(1) or (2); or
(c)regulation 17(1), (2) or (4),
may carry out such work if he does so under the supervision of a person who has obtained the requisite qualification and with a view to obtaining that qualification.