"Were 3 direct debits paid out between 11th January and 11th February (assuming the 11th is your statement date).

Remember that the 3 direct debits must be paid out in your statement month (not calendar month). So, for example, if one normally pays out on the 10th or 11th of the month then it would be paid the next working day on the 12th February and not qualify as one of the three needed for your February interest.

What dates did your DDs actually get taken? "

As you say, looks like the start and end date of the statement month in also included.
My statement date falls on 06th. So last months statement=06 September to 06 October.
Received payment of over £750 only yesterday (last day of statement month). This morning when I checked app (07 October) interest of £7.29 had been added to account.