Nine in ten people in the UK who buy a car use finance. The sector is the second largest lender to consumers in the UK after mortgages. But the industry is bracing itself for a Supreme Court judgement which could see lenders forced to pay out tens of billions in compensation for selling deals with 'secret' commissions. File on 4 Investigates hears from industry insiders and those who tried to raise the alarm on a scandal that some believe could rival PPI.

Reporter: Helen Catt