Okay, two things to note here:
1) This isn't actually Alt-J, it's their lead singer Joe Newman. I just put that in the title because of clickbait.
2) This song is more than a year old. I only just recently discovered it, and there's a good chance you'll find yourself in the same boat.
"101" was released by Ninja Tune and appears on London-based Portico's 2015 album Living Fields. It's a slow-burner with unmistakable vocals, and I dare say comes across as more interesting than much of Alt-J's most-recent album. Get your hands on the full album here.