Britain's biggest banks have so far paid around 600m pounds for mis-selling interest rate hedging products, according to the UK's Financial Conduct Authority (FCA).The banks have set aside £4bn to compensate small firms.Banks including Barclays, HSBC, Royal Bank of Scotland and Lloyds Banking Group are on track to meet a deadline to review all cases by the end of May, the regulator said.The interest rate hedging products were designed to protect smaller companies against rising interest rates. However, when rates fell, they had to pay tens of thousands of pounds in bills. Companies were also hit with penalties to get out of the deals, which many said they were not told about. RD