Well looking at BT TV Player catch up the last episode added is
Killer Attitude (Northwest Airlink Flight 5719) which appears to be series 16 episode 1 according to BT Player and National Geographic (natgeotv.com) but wikipedia lists the episode as series 17 episode 1.
There seem to be multiple episodes showing from multiple series on National geographic at multiple times - if the episode broadcast is different to the one in the schedule that generally is down to the broadcaster supplying the wrong information in its schedules. Of course it is possible that late schedule chages might occur.
From what I can see the latest (new) episodes are appearing on the BT player catch up so if you have recorded an old episode you can use the catchup service to view the latest.