(1) Where a person -

(a) becomes a self-employed contributor subsequent to being an employed contributor in any contribution year, or

(b) is a self-employed contributor prior to being an employed contributor in any contribution year,

and is a person to whom paragraphs (a) or (c) of section 18(1) applies, self-employment contributions shall be payable by him at the percentage amount or the amount specified in the said paragraphs, whichever is the greater. 

(2) In the case of a person to whom this article applies, the number of contribution weeks in respect of which self-employment contributions shall be regarded as having been paid, shall (subject to the appropriate amount of self-employment contributions having been paid and, notwithstanding the provisions of section 18(1) (e)), be determined by deducting the number of contribution weeks in which the person was an employed contributor from 52, and the number so determined shall be regarded as the number of contribution weeks in respect of which self-employment contributions shall be regarded as having been paid.