Assuming you are due to receive only one more pay cheque in this current financial year it is a simple matter to determine how much tax you should pay. Take the gross pay to date from your P45 and add £940 (the coming month’s salary) to that figure. This is your gross pay for the year. Reduce this figure by £8,105 (your tax free personal allowance, assuming you have just the standard allowance). This is your taxable pay. Take 20% of that figure (the tax due). Compare that figure from the tax deducted to date from your P45. The difference is the tax due to be paid (or refunded) in your March pay packet.
So, for example, if you have earned (gross) £12,000 over the first ten months of the financial year, your P45 should show £12,000 gross pay, and about £1,050 tax paid. Your final month’s pay will take your gross pay to £12,940. You have a tax free allowance of £8,105 for the year. This means that tax will be due on £4,835 making your tax bill for the year £967. You’ve already paid £1,050 so you will be due a refund of £83.
As far as NI is concerned you can earn about £633 per month before NI becomes due. Above this you pay 12%. So you will pay 12% on £307, which is about £37. Unlike income tax NI is not cumulative. It is charged on each week’s (or month’s) income in isolation and cannot be reclaimed in the same way as income tax.
Of course all this would be complicated somewhat if your earlier job took you into the 40% tax bracket.