Working in the Netherlands

My son worked for about 6 months at airport in the Netherlands by an English employment agency, with branches in the Netherlands (all in the same tax year)

He earned a toal of 9895 euros of which 1938.27 was called reimbursed expenses (of which my son said was overtime payments) he paid dutch tax of 23.95 and uk tax of 798.90 along with national insurance of 550.59.  These figures are in euros with one sterling payment of gross 469.84 tax £38.20 ni £24.18 as a seperate payment.

He was given a P45 showing £6511.65 gross earning s with uk tax of £376.88.

Having queried this P45 with their payroll department, they say that the equilivant  sterling earnings are £651.65 with uk tax of £376.88 and NL tax of £324.23.

Can anyone tell me if this is correct and how do we get the Netherland tax back as he will not have earned enough in the tax year to pay any tax in the UK and should  be refunded.

Many thanks

Mary