A diesel's speed and idle is controlled by varying the length of injector timing and unlike a petrol engine it doesn't use a butterfly to control the amount of air entering, within reason, an intake leak has little if no affect on its speed.
On the CRD there is a throttle cable running to an electronic throttle controller at the rear offside of the engine bay. If you plug in a basic scanner with the ignition on and the engine off, you can check the unit and that the cable is returning properly, it should read 0% and gradually raise to 100%and back again as you slowly push the pedal down and release, test it with the engine bay hot and cold.
While you've got the scanner on check that the temperature sensors around the engine are making sense, these are what the ECU's are using to calculate their actions.
