Not Returning for a Series 3
I'm sorry to report such tragic news but ITV announced that THE BLETCHLEY CIRCLE has not been commissioned for a third series and won't be returning.
http://m.digitalspy.com/british-tv/news/a564880/the-bletchley-circle-axed-by-itv.html#~oBL6PlADnxxfyK
It's such a shame. As an American guy, I loved this series since (oddly enough) I was a fan of Nancy Drew and Marple. This was an excellent series that should have been shown a bit more love :\
Hopefully ITV will reconsider this decision but as it stands now, no third series.