The important thing to check is whether the total tax deducted so far in this tax year is correct. If you find that the figure is correct, you must have been paying too little tax in previous months, so there's nothing you can do about it. However, if the figure is too high it suggests that you've had too much deducted this month.
I'm assuming
(i) that your pay slip shows the total taxable pay, and total tax, deducted for the current tax year (most pay slips do) ; and
(ii) that your tax code (which should be on your pay slip) is 603L. (Nearly everyone has that code for this year. If yours is different, you need to speak to your accounts people) ; and
(iii) that you don't earn enough to pay 'higher rate' tax (i.e. not more than �40,835 p.a.)
Start with the figure for total taxable pay (for this financial year). If your pay period ended last weekend, subtract �4874.42. If your pay period ends this coming weekend, subtract �4990.48. Divide your total by 5. That figure should be the total amount of tax you've paid so far this year.
Chris