SolvedSTSM152030 | Compliance: Stamp Duty interest: Calculation of interest
From HM Revenue & Customs · Stamp Taxes on Shares Manual
The rate of interest applying to late payment of stamp duty is the rate applicable under FA89/S178 and is set by the Treasury.
No interest is payable if the interest totals less than £25. Calculations of interest are rounded down to the nearest multiple of £5.