MD83 Elevators
 

Woudl somebody please explain how these work....sat behind one at the holding point and they just seemed to be flopping about!

I assume that they take a neutral position on take off but how are they actuated, they were split and just "flapping about" in the wind.

First I'm an Avionics Engineer not Airframe however; The MD elevators are not powered in any way and are controlled by tabs actuated by cable directly from the control columns, therefore when stationary it is normal for the elevators to not appear connected to anything, they only fair with fwd speed and therefore airflow.
The exception to this is the stall condition when a small hydraulic actuator drives the tab to give full a/c nose down elevator.