Holiday Pay/WTR expert

Hi, urgently need advice from a payroll expert who is able to provide advice on whether holiday pay calculations are correct and comply with the law.

Task 1 - 500 weekly payslips - Nov 2012-Nov 2019 - holiday pay was not paid on overtime and bonus. Need to know what payments employee entitled to in backpay.

Task 2 - employee not paid holiday pay on overtime to Dec 2018. New policy introduced Jan 2019 whereby overtime would accrue holiday pay. Started to make payments in Jan 2019 to correct Oct 2018-Dec 2018 shortfall by adding up overtime over previous 12 weeks and dividing by 12, then paying Jan onwards. Need to identify some way that Jan corrections were underpayments in how they were calculated.

MUST HAVE KNOWLEDGE OF UK RULES ON HOLIDAY PAY AND OVERTIME