From the DWP site
http://www.direct.gov...alInsurance/DG_190048
From April 2011, your Class 2 National Insurance contributions payments will become due on 31 January and 31 July, the same as a Self Assessment tax bill. You pay Class 2 National Insurance contributions either monthly or six monthly by Direct Debit – follow the first link below for more information about payment dates.
You pay Class 4 National Insurance contributions when you pay your Income Tax.
How to pay self-employed Class 2 National Insurance contributions Opens new window
Find out more about Class 2 National Insurance contributions Opens new window