Wot's this ere thing in the motor box?

Thermal trip, overload trip, spider, ... A picture would help.

If it's really a *start* capacitor as opposed to a *run* capacitor, there will be something to switch it out of the circuit once the motor has started, such as a centrifugal switch.

I had thought it might be thermal protection, but it's not up against the motor as such a device ought to be, surely. I can't provide a photo, but appears on the exterior of the box to be a roughly 12mm diameter threaded black plastic collar with a pin in the middle. This pin has a tiny hole in the middle of it. Overall it resembles a coaxial socket from an old style TV. The pin doesn't appear to move as I would have expected for re-setting purposes.And behind that as I've said is a mains voltage microswitch in series with the incoming live wire.