(1) An employer shall not permit an employee to work, in each period of 7 days, more than an average of 48 hours, that is to say an average of 48 hours calculated over a period (hereafter in this section referred to as a ''reference period'') that does not exceed -

(a) 4 months, or

(b) 6 months -

(i) in the case of an employee employed in an activity referred to in paragraph 2, point 2.1. of Article 17 of the Council Directive, or

(ii) where due to any matter referred to in section 5, it would not be practicable (if a reference period not exceeding 4 months were to apply in relation to the employee) for the employer to comply with this subsection,

or

(c) such length of time as, in the case of an employee employed in an activity mentioned in subsection (5), is specified in a collective agreement referred to in that subsection.

(2) Subsection (1) shall have effect subject to the Fifth Schedule (which contains transitional provisions in respect of the period of 24 months beginning on the commencement of that Schedule).