The official payment date should be used as that is the date the sum was declared payable by the company, no matter that you had to wait longer to receive it. This will cause difficulty to those UK taxpayers who plan to look at the csv transaction download to figure their taxes due in the year to 5 April.
Here they say more. This agrees with what I have written above. It is the declared payment date that counts.
The timing and tax point of dividend declarations | Intouch.
Question 1: When are dividends taxed? Is it when they’re paid, or the date they’re declared?
Neither of these answers are correct. A dividend will be included on your tax return, according to the date the dividend was declared as becoming payable. The date it was paid is not relevant.