Employer Making Employee Calculate own Holiday Pay

Has anyone come across this. Friends of mine have a casual labour zero hours contract job. They are employedby a large well known firm whose name I wont mention. Pay is paid monthly based on hours worked. Until recently Holiday Pay was correctly shown as a separate item on the payslip. They have now been told due to "new government rules" they must calculate their own holiday pay and submit it. Has anyone heard of this. Understandably they have no idea how to do it - it's bad enough for us who  are dealing with it on a daily basis.As I am a payroll agent they have asked for my help.