Regulations came into effect on 22 July 2009 to introduce changes to the way that HMRC calculates its interest rates. HMRC will recalculate its rates after the September Monetary Policy Meeting (MPC) using the new formulae provided in the regulations. This should result in a change to some of HMRC's interest rates even if there is no change in the base rate announced by the MPC.
The harmonised rates will be based around the Bank of England base rate and will change in response to any changes announced by the Monetary Policy Committee 13 working days later.
Interest charged on late payments of tax will be the Bank of England base rate plus 2.5 and for interest paid on overpayments, the rate will be the Bank of England base rate minus 1.
Further details are available here