Simple BPP Question regarding overpaid tax

Hi all, i seem to be struggling with this question from the BPP course book, as i thought the answer was £1275

Milton Ltd paid its CT liability for the year ended 31 December 2024 on 31 July 2025. The final accounts showed an overpayment of £120,000 tax and HMRC paid this, along with the relevant interest to Milton Ltd on 31 October 2025.
How much interest is payable by HMRC to Milton Ltd?
– £nil
– £425
– £1,275
– £850
Correct
The correct answer is: £425
Normal due date (NDD) for year ended 31 December 2024 is 1 October 2025.
Original tax for year ended 31 December 2024 was paid on 31 July 2025.
Interest runs from the later of these two dates, i.e. 1 October 2025.
So, a month’s interest is payable at 4.25%, i.e.
4.25% × £120,000 × 1/12 = £425
The answer £nil assumes there was no overpayment.
The answer £1,275 is three months’ interest (i.e. from the payment date).
The answer £850 is two months’ interest (i.e. from the payment date to the due date).

thank you 🙂