getting closer...
Thanks Gaunty,
I had a look at the link. With the info there and what I have previously read, it appears that the autopilot application to the Stab trim uses electrically powered servo motors. It doesn't really say how the stabiliser is then moved, but I guess (stab in the dark!) that these servomotors would not be there to power a hydraulic actuation. (hydraulic pressure is already available in the aircraft and probably doesn't need a new local source). Rather, they would more likely be used to power a mechanical actuation. Such as a nut and bolt type system where the bolt screws through the actuation arm, to in turn move the stabiliser.
So it seems like, the autopliot control of the stab trim (to my thinking anyway) is an electronic/mechanical actuation. Assuming that the 73 and 76 systems are similar in this instance.
If anyone else has anything else to add, please feel free to.
Bort.