One other pointer to the notion that Inversions is a Culture novel, and that the Doctor is an agent of Contact:
**SPOILER WARNING**
At the end of the book, when the Doctor has decided to 'return to Drezen', there is an obvious pickup by a spacecraft: flashing lights in the sky (attributed to chain fire, but much brighter and louder), harsh winds that come and go suddenly, etc. This in and of itself isn't as telling as what the Doctor told the captain of the native sea vessel when she declined to eat dinner with him that night, "citing an indisposition due to special circumstances." Special Circumstances is, of course, the name of the branch of Contact that handles first-time interaction between the Culture and technologically inferior civilizations.