Self employed - self assessment and agency's payslip

Hi.. Question related to building trade self employment (kind of), where working via agency and getting paid via payroll company, which collects a fee for each payslip. Apologies if question seems simple, but I just got mixed up regarding which amount I would use for self assessment where I need to specify gross income. Example weekly Payslip: Contracted sum: £780.00 Agency fee: £16.00 Gross pay: £764.00 Deducted tax (CIS): £152.80 Payment to bank: £611.20 When doing my self assessment I think I need to specify top sum (contracted sum - £780.00 ) as my gross income (before taxes and expenses). I will specify paid tax separately and add agency's fee as expenses. is that the correct way? Or I need to specify only taxable amount £764.00 as my income before tax. If that's the case, can I still add agency's fee as expense? Second way does not seem right, but I got myself confused now. Perhaps someone with similar experience can share what do they specify as income.

Hi

In your self assessment tax return, you would use the contracted sum as the gross, the agency fee declared as an expense and the cis deduction of tax paid.  
This will allow you to declare gross profit and expenses, to arrive at the net taxable profit and the tax paid.

Thank you