It definitely wasnt the seatpost guts. I slathered the post in grease, put it back in the frame but it still creaked. A bit of bedtime googling told me that regular grease and carbon is a big no no because it can make it swell. In the morning I checked it and the creakiness has gone. Took it out and wiped off all the grease and did the same to the frame. The creak was still gone. Hurrah!
Tl;Dr
It creaked, I greased it, it doesn't creak.