I would slightly disagree that the clutch is doing nothing on a motorway!
But if it was slipping you should notice it, put the car in top gear at a fairly low speed and floor the throttle... if the engine speed picks up faster then you car speed then clutch is shot (though unlikely in a 17k mile car unless it's been driven really badly)
Rattling from when the clutch is depressed is normally the clutch thrust bearing... again not likely in a 17k mile car.
thinking about it are you 100% sure on the actual milage of this car?