 If the sole-trader reaches the threshold of £33,540 then he will have to pay Class 4 NIC this tax band is charged on a percentage.Also sole traders must also pay Income Tax they do this by filling in the self-assessment form which also allows the Inland Revenue to calculate Class 4 NIC; this is simple to fill the information needed is usually costs, sales and profits; if turnover is above £15,000 you may have to keep a detailed profit and loss and balance sheet.Basic records will normally include