Revving

Air leak around the plenum or pipework. Throttle position sensor out of adjustment. Incorrect speed input into the ECU. Stepper not being driven shut by the ECU (unlikely). Try clamping the air feed pipe to the stepper and see if the idle drops to about 500 rpm? If it does not you have an air leak. If its not an obvious air leak you need to borrow / buy an ECUmate or a RoverGauge cable to look at the ECU inputs and stepper status.

Many thanks folks and especially the offer of the loan of ECUmate, all sorted now, fingers crossed.