Backdated Pay, tax wrong?

throughtheblue

I'm an NHS worker and today noticed my e-payroll had been updated for this months pay, as I'm expecting a backdated pay rise from April 22 - December (9 months), I've been keen to make sure everything adds up. 
My extra pay in arrears was £1631.77 (before tax), however, the amount extra I've received after tax is roughly £850. Obviously, I knew the amount would be taxed retrospectively, but I was imaging an extra £1200 after tax. What's the best way of resolving this to make sure it's correct before over complicating it? Tax code is  S1271L, so no problems there. 

Dazed_and_C0nfused

Why do you think it's the tax that's wrong?

Has it taken you into higher rate territory?

Might be the pension contribution with the recentish NHS changes.

throughtheblue

My pension contribution  this month went up an extra £110, so roughly £10 extra per month retrospectively. Student Loan went up £140, so £14 extra per month retrospectively, NI insurance went up an extra £150, so an extra £15 per month retrospectively. These look correct, but my tax has gone from £200 per month, to £518 this month, which seems wrong. 
Instead of applying the deductions retrospectively, it looks like they've added the arrears to my gross pay, and taxed the gross pay as if I wold earn that every single month. Is that possible?

tightauldgit

From my experience, yes, when you get an unexpected bump in pay like that they tax you as if that's going to be what you earn every month. 

If you do nothing it will iron itself out at the end of the tax year and you'll get a refund. You can probably contact HMRC and tell them the error but given the date and how close we are to the end of the tax year it probably won't be resolved any quicker.  

Dazed_and_C0nfused

I suspect the tax is correct as it will be based on the facts.

You have earned an extra £1,631.77 (less any net pay pension contributions).

You can use this calculator to check last month, this month and next month's figures.

http://payecalculator.hmrc.gov.uk/PAYE0.aspx

molerat

£1631 x 20% = £326, £518 - £200 = £318 so the increase in tax is just under 20% of the back pay which is correct.  The other bits add up to £400 so a total of £718 leaving £913 net extra which is in the ballpark

throughtheblue

Thanks for the input. I think it looks correct on reflection. 
My increase in pay after tax is roughly £96 per month, and the backdated pay is from April - Dec, so 9 months. £96 x 9 = £864, which is roughly the extra I have received, this month compared to last. 
I think seeing the sum in arrears is what confused me, going from £1660 to £860.