You don't pay anything back until you earn over £21,000 and you then pay 9% ONLY on what you go over.
For example.
If you earn a salary of £21,001 per year. You will pay back 9 pence that year, because you've only gone above £21,000 by 100 pence.
If that salary stays the same for 30 years, then in total you will repay £2.70 before it gets wiped off by the government.
Hence, the worry over student loan repayments is over the top.