We all have to start somewhere!

Chrisjo: what would be the explanation for this kick-starting? Perhaps the gears are gummed up and they just need a bit of extra 'push' to get turning again?

It's not the gears that get gummed up, it's something inside the motor that makes it stick after a prolonged period of disuse.  The next step if the kick-start technique fails is to strip down the gearbox, remove the motor, apply power to the terminals, and spin the shaft with your fingers. Only if this fails can you be sure the motor is dead.  There are gearbox dis-assembly / re-assembly tutorials in other threads here and elsewhere online (Youtube), but really all you need to do is take a photo of every stage of the strip-down so that you can reverse the process. Once you know how they work it becomes almost routine. Great care must be taken when first opening the two halves of the gearbox/motor housing, it's at this stage that bad things can happen, like springs flying off into another dimension.

Once a motor has been re-started it will almost always run smoothly thereafter, but a drop of lubrication with a thin mineral oil such as WD-40 where the shaft enters the motor housing never goes amiss, along with a touch of silicone grease on each of the gears.  You'd be amazed how much hair and carpet fluff there is inside the gearboxes of some of the stuff you buy second-hand, they do appreciate a good clean-out.