Self Employed National Insurance is a flat weekly rate National Insurance, irrespective of how much you earn from your self employment. So if you earned �30 or �30,000 since January, you still have a liability to pay the �22.00 bill (10 weeks @ �2.20). In the 2007/08 tax year the weekly rate was �2.20. In year 2008/09 year the weekly rate is �2.30.
If you are both employed (PAYE) and self employed, you must pay both types of National Insurance. However if your self employed income is expected to be low, you can claim Small Earnings Exception. This must be applied for as it is not automatically awarded. The claim form is here on the HMRC website..
http://search2.hmrc.gov.uk/kbroker/hmrc/forms/ viewform.jsp?formName=CF10
Print off the form and send it to the address given.