Who knows when it'll be on TV this side of the channel but a third series of Spiral has been filmed, this time with 12 episodes rather than 8.
You can read more about the new series (in French) on this website. On the second page there's a comment about the success of the first two series, overseas, and says that's it's the first time the BBC has bought a French series since the 1960s and the realistic nature of the series strikes a cord with Britons (paraphrasing a bit, I hope correctly).
Engrenages (Spiral) as been so successful in France that three more series have been ordered.
The official Canal+ website for series 2 is here.