You will need to assess the amount you've earned from 6 April 2006 to 5 April 2007.
You pay tax at 0% on the first £5,035; 10% on the next £2,150; 22% on the next £31,150 and 40% above that.
If you find you've paid too much tax, you can claim a refund (this includes where you've had tax deducted at source on bank accounts - although the 22% basic rate is 20% on interest income).
You won't, however, get any refund on National Insurance contributions paid.
To conclude: As JPreston said, if you're only working for part of the year, it's likely you'll pay too much tax and be entitled to a refund - this should happen automatically, but I wuldn't count on it. If you're working for the full year, you should expect that anything over the first ~£400 per month is taxed.