Self-employed individuals generally pay class 2 national insurance by monthly direct debit, or when the quarterly bill arrives from the Tax Office.

From 6 April 2011 the class 2 NI for the tax year will be due in two equal instalments on 31 July and 31 January. The Tax Office will send out separate bills in April and October for two instalments of class 2 NI.  If you already have a monthly direct debit set up to pay the class 2 NI, those direct debits will be suspended from April 2011 and will resume in August 2011. You will also have the option of paying by direct debit in two instalments in July and January.

Changes to class 2 NI
http://www.hmrc.gov.uk/payinghmrc/news-nic2-payments.htm