12(1)Any contract of employment under which a shop worker who satisfies the conditions in paragraph 2(2)(a) and (b) above was employed on the day before the commencement date is unenforceable to the extent that it—
(a)requires the shop worker to do shop work on Sunday on or after the commencement date, or
(b)requires the employer to provide the shop worker with shop work on Sunday on or after that date.
(2)Except as provided by sub-paragraph (3) below, any agreement entered into after the commencement date between a protected shop worker and his employer is unenforceable to the extent that it—
(a)requires the shop worker to do shop work on Sunday, or
(b)requires the employer to provide the shop worker with shop work on Sunday.
(3)Where, after giving an opting-in notice, a protected shop worker expressly agrees as mentioned in paragraph 3(1)(b) above (and so ceases to be protected), his contract of employment shall be taken to be varied to the extent necessary to give effect to the terms of the agreement.
(4)The reference in sub-paragraph (2) above to a protected shop worker includes a reference to an employee who, although not a protected shop worker for the purposes of that sub-paragraph at the time when the agreement is entered into, is a protected shop worker on the day on which she returns to work as mentioned in paragraph 10 of Schedule 13 to the 1978 Act (maternity).