SolvedCH140180 | Interest: Rates of interest
From HM Revenue & Customs · Compliance Handbook
You must check whether, and from which date, the FA 2009 interest rules apply to the tax or duty you are dealing with. See CH140160 for full details.
The Treasury sets the rates for late payment interest and repayment interest and the rates appear in Regulations.
SI2011/2446 came into force on 31 October 2011 and sets the current rates which are linked to the Bank of England base rate by means of the following statutory formula:
Late payment interest = Base rate PLUS 4%
Repayment Interest = the higher of, (a) Base rate MINUS 1% or (b) 0.5%