"The Company reserves the right to change your shift pattern and pay the appropriate premium when applicable, and you may be required to work overtime at the Managements discretion."
"13. At the end of the day we are satisfied that the applicant was employed to work the nightshift on such occasions as was required by his employer. That is clear in our view from the agreement at page 28 at seq. It is also clear from his letter of appointment: page 8.