CH146340 | Interest: Repayment interest: Special provisions for start dates: Mortgage interest relief at source (MIRAS) - example

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.

Kate has a home income plan which has been in existence since before 9 March 1999.

Kate makes payments during the year 2012-13 without deducting tax from those payments.

The repayment interest start date is 31 January 2014.

If HMRC pays Kate an amount equal to the amount that she could have deducted from her payments

before 31 January 2014, then no repayment interest is payable,
after 31 January 2014, then repayment interest is payable from 31 January 2014 until the amount is refunded.