I got a confirmation email after I'd done the three fill-ups for this offer, and if I recall correctly, it gave 25/01 (today!) as the day by which I would receive the 1600 bonus points. I didn't yet have them as of a couple of hours ago.
I also received the confirmation email after my 3rd fill up but nothing after the 4th which I thought they might have sent.
No mention of a specific date in the email either, just the allow approx. 3 weeks.
I would expect to see them before the 28th as that would be 4 weeks after the 31st December