Calculating Redundancy Payment for period of Temperory Employment

One of my collogues has been offered a redundancy payment. He has been with the company 6 years with the first 18 months as a temperory employee. The company is calculating the redundancy lump sum payment on the period when he was a permanent employee. However to calculate the tax free portion should they take his initial start date into account.
Thanks
Nadd

his statutory payment should be calculated on the 6 year period.