Further to my previous messages, I've since built the circuit shown in that video, and it worked (the motor is powering the dinosaur):
Or rather, it starts off working, then stops.
I swapped the dino motor with a regular one, and that works (and continues to work regardless), and also put an LED in place of the motor, which also worked, but upon plugging the dino one back in it still didn't work.
At this point, I figured that I'd killed the motor somehow, but placing an AA battery across its contacts made it spring back into life - and in fact seems to temporarily repair it as part of the circuit, but only for two or three runs (of 5 seconds each) then it stops working again.
I've tried it connected to both the 3.3 and 5v pins, but get the same issue both ways - it works for 2 or 3 runs, then stops working again until it's reset using a battery. This only happens with the dino motor, but not the other one.
Does anyone have any ideas?
(PS, silly project is silly)