Hello HLock,
Welcome to the Community page,
So i presume this is CIS, to record the invoice you would enter the amount of the labour on one line and add another as a negative relating to the 20% so if its 1000 you would enter -200 on the second line which would make the due amount 800. when you record the coming in, you record it as business income, to record the amount that is suffered you would record that in the actual self assessment form on HMRC's website.
