Student Loan interest

Hey everyone,
So I have a little dilemma regarding the Student Loan. The pay of the interest is like the one for the loan itself? I mean you start to pay it once you earn more than £ 21.000 and it's taken from the amount which exceeds the threshold or from your whole salary ?

The interest is treated exactly like the loan. It is added to the loan amount and the whole sum is subject to the same terms. You start making repayments on the whole thing, only once you earn £21,000 or over.