It seems Poland might be the first country to air the final series. Elephants Can Remember, The Big Four, The Labours of Hercules, Dead Man's Folly and Curtain have all been given air dates, one for each month up to and including January 2014.
Air dates in Poland are:
Elephants Can Remember - 6 September 2013
The Big Four - 4 October 2013
The Labours of Hercules - 1 November 2013
Dead Man's Folly - 6 December 2013
Curtain - 3 January 2014
See the Polish schedule (in Polish) here: http://www.alekinoplus.pl/alekino_wydarzenia-20130905-detektywi.html. And for those of us who aren't fluent in Polish, see this Google translation.
What this means is that the series is definitely through post-production and has been released for the international market. Surely ITV can't be far behind?
Thanks to Tom, a reader of the blog, and ueetba, an IMdB user, for the information!