If you expected your self-employed profits for 2009/10 to be low, you may have reduced your tax payments on account for 2009/10. Once your 2009/10 tax return is completed you may find you owe extra tax for 2009/10. In this case

The Tax Office will send you a tax demand for tax due on 31 January 2010 and 31 July 2010. Please don’t be offended by the tone of this letter, it is not very tactful.

 How to pay self-employed tax

 http://www.hmrc.gov.uk/payinghmrc/selfassessment.htm