Shut Down Deductions

Hello. Our business has a 4-day shut down period in December (between Christmas and New Year) whereby employees are unpaid, however, rather than deducting the 4 days pay from employee's pay in December, the cost is spread out over 12 months. This amount is included as a "sacrifice" as its a reduction of pay (as opposed to a deduction to pay) for the cash, however, if we deducted the time in December, this could inadvertantly mask any underpayment of NMW. Is there any advice on how to approach either the calculation, or how we deduct the shut down?

Hi,
Salary sacrifice will always reduce a worker's minimum wage pay. The worker is in essence giving up that portion of their pay in exchange for something else, so it cannot be included in any minimum wage calculations. 
Any determination would depend on the arrangements in place but where a salary sacrifice is in place, the worker is no longer has entitlement to that amount of pay under their contract. Therefore, their pay for minimum wage purposes is reduced accordingly.
For more information on salary sacrifice arrangements please refer to the following guidance:
Salary sacrifice
When considering the working hours for which the minimum wage must be paid, you may find the guidance on absences useful. How the absence is treated depends on whether the worker is performing salaried hours work, time work, output work or unmeasured work. You can see more information here:
Working hours for which the minimum wage must be paid
Thank you.