I've discovered why my port side trim tab actuator wasn't working. The motor is seized due to water ingress. See the puddle on the floor in the bottom right corner of the pic below? All that came out of the actuator when I opened it up.
Luckily, despite looking crappy, the gears are all fine and turn the ram without excess friction or sticking, which means I can clean them up, relube them and re-use them.
So I'll order a replacement motor and see how it goes from there.