This may help
http://en.wikipedia.org/wiki/National_Insurance_(United_Kingdom)#Contribution_classes
Self employed people pay class 2 contributions:
"Class 2
Class 2 contributions are fixed weekly amounts paid by the self-employed. They are due regardless of trading profits or losses, but those with low earnings can apply for exemption from paying and those on high earnings with liability to either Class 1 or 4 can apply for deferment from paying. While the amount is calculated to a weekly figure, they are typically paid monthly or quarterly. For the most part, unlike Class 1, they do not form part of a qualifying contribution record for contributions-based Jobseekers Allowance, but do count towards Employment and Support Allowance.
Self employed people also pay NI (class 4) on profits:
"Class 4
Class 4 contributions are paid by self-employed people as a portion of their profits. The amount due is calculated with income tax at the end of the year, based on figures supplied on the SA100 tax return.
Contributions are based around two thresholds, the Lower Profits Limit (LPL) and the Upper Profits Limit (UPL). These have the same cash values as the Primary Threshold and Upper Earnings Limit used in Class 1 calculations.
No class 4 NICs are due on profits up to and including the LPL.
Above the LPL, up to and including the UPL, class 4 NICs are paid at a rate of 9% (tax year 2011-12).
Above the UPL, class 4 NICs are paid at a rate of 2%.[10]
Class 4 contributions do not form part of a qualifying contribution record for any benefits, including the State Pension, as self-employed people qualify for these benefits by paying Class 2 contributions."