Andraste

That works for me - especially considering that the massive discrepancy between the books and the series isn't Gallifrey (since Eight does have the Matrix in his head and it's just about possible he found some way to transplant it into cloned bodies or something on a new Gallifrey): it's Sarah. There's no real way to square "I've regenerated half a dozen times since I saw you last" with him not only seeing her two regenerations ago but his companion going to live with her! I mean, you'd think one of them would remember a little thing like that...