I wouldn't have minded if I had guessed it myself. But one little line in the Guardian Guide prompted me. All it did was make me think of something I hadn't thought of before, but it felt like a spoiler: "The Doctor comes closer than ever before to returning to Gallifrey," or some such.
And there it was: “They” from last week had to be the Time Lords.
But why? Why did they do it? Why put the Doctor through that, just to get him to Gallifrey? And also, how? of course: how can he get to Gallifrey when it’s supposed to be locked away in some pocket universe?
And titling: why was it called “Heaven Sent”?
Great episode, by the way. Best of the season. Indeed, I predict a Hugo.
And I expect we’ll find out some of the answers next week.