just found regulations on opsi.gov.uk
"young worker" means a worker who has attained the age of 15 but not the age of 18"
" Maximum working time for young workers
5A. - (1) A young worker's working time shall not exceed -
(a) eight hours a day, or
(b) 40 hours a week.
(2) If, on any day, or, as the case may be, during any week, a young worker is employed by more than one employer, his working time shall be determined for the purpose of paragraph (1) by aggregating the number of hours worked by him for each employer.
(3) For the purposes of paragraphs (1) and (2), a week starts at midnight between Sunday and Monday.
(4) An employer shall take all reasonable steps, in keeping with the need to protect the health and safety of workers, to ensure that the limits specified in paragraph (1) are complied with in the case of each worker employed by him in relation to whom they apply."