Hi Helen

I have always used base salary only to calculate any accrued and unused holiday pay. Car / shift allowance are usually on top of or in addition to salary and would not be included in base salary for reckonable calculations.

Thanks
Fionnuala

It will depend on how holiday pay is calculated during employment. There should be no difference in the calculation when they leave - either to make a payment for accrued but not taken holiday or a deduction for days taken over and above those accrued at leave date.
If shift pay is included in the calculation during employment then it needs to be included for in any calculation when leaving. If, during employment, holidays are only paid based on basic rate then you can use basic rate for leaver calculations.
In regard to car allowance I have never known this to be included in any holiday pay calculations either during employment or on termination. Car allowances are usually a completely separate issue, awarded as a yearly/monthly amount and not related to any hourly rate and therefore unrelated to any holiday pay calculations.
Also, if holiday pay is variable (e.g.due to overtime also being included) then again this would be the same calculation for payment for days accrued but not taken on leaving date. If it is variable and a leaver has taken more than accrued then work backwards from the last holiday and deduct the amount actually paid for the number of days that were taken and not accrued (e.g. if 3 days taken over accrual then whatever was paid for the last 3 days holiday taken is the amount owed by the employee for deduction from final pay).