18(1) The Inland Revenue may by regulations make provision so that where–
(a)an earner, in respect of any one or more employments of his, is treated by regulations under section 2(2)(b) above as being self-employed; and
(b)in any tax year he has earnings from any such employment (one or more) which fall within paragraph (b)(i) of subsection (8) of section 11 above but is not liable for a higher weekly rate of Class 2 contributions by virtue of regulations under that subsection; and
(c)the total of those earnings exceeds £9,568,
he is to be liable, in respect of those earnings, to pay a Class 4 contribution.