The following assumes that your bonus is paid before the end of the current tax year:
If your total income (including the bonus), during the 2007/08 tax year, doesn't exceed �39825, you'll pay 22% tax on the bonus. If your income, before receiving the bonus, already exceeds �39825, you'll pay 40% tax on the bonus. If you were receiving less than �39825 before the bonus, but the bonus takes your income over that amount, you'll pay 22% tax on the part that takes you up to �39825 and 40% tax on the rest.
You'll also pay National Insurance on the bonus. For any part of the bonus that takes your total annual income up to �34840, you'll pay 11% NI. For anything above that, you'll only pay 1% NI.
I could type out loads of complicated details about how to calculate your take home pay but it's far simpler to suggest that you use this online calculator:
http://www.thesalarycalculator.co.uk/salary.ph p
Chris