Centrica's British Gas and Sainsbury's have been hit by a new mis-selling scandal and are expected to return £500,000 to more than 4,000 affected customers.British Gas has, according to Sky News, already notified industry regulator Ofgem, which is apparently satisfied with the company's proposal to credit customer accounts with an average repayment of around £130.Sources revealed that the companies had overstated potential savings from switching to another tariff from Sainsbury's Energy and British Gas by using inaccurate quotes for the following year's gas and electricity bills.Sainsbury's Energy has operated in partnership with British Gas, which is part of Centrica, for several years.Ofgem is expected to confirm the compensation package in a public statement in coming days.OH