Whether you are employed or self-employed, there is a limit to the amount of NI you need to pay each tax year. The limit for 2009/10 is £43,875. An additional 1 per cent is due on all earnings above this limit. With your salaried job, you will be paying class 1 NI at 11 per cent on your earnings up to the limit and on your profit share, you will be paying class 4 NI at 8 per cent.
This may well mean that you are paying more NI than you need to. Usually HM Revenue & Custom's NI office will contact you if it thinks you have overpaid.
If not, I suggest you write to your tax office with details of the NI you have paid on your employed and self-employed income. State clearly your NI number and tax reference.