A flexible working hours agreement is in place but with a caveat in the agreement of "you may be required to work beyond these hours as the Company shall require…with no additional pay" ; the company now has a big project for next year and two daytimes a week will need to shift to nighttime work (11pm-4am). 
Is this enforcable within the flexible working hours agreement?

If the hours are now night-time shouldn't the working hours be limited to 8 within a 24-hr period?

Thanks for any help….