I think that would depend on how you have the accounts set up.
I tend to treat it like any other item paid for, for personal use, and CR Bank (if paid by chq) and Dr Drawings, as like a tax bill the NI contribution is the reponsibility of the individual and not a business expense (albiet, the result of running his/ her business).
Th only way NI is a business expense, is when it is paid as part of an employees gross wages and the additional employers contribution