Official rate of interest
From 6 April 2010 the official rate of interest applying to interest-free or low interest loans made to employees will be 4%. The current rate is 4.75%. Employees are generally liable to tax on a benefit equal to the amount of interest, calculated at the official rate, where the balance of the loan exceeds £5,000 at any point in the tax year.
Internet link: HMRC beneficial loan interest rate