AaronMac Before firing the "parts cannon" at it, I'd recommend confirming the throttle position sensor is adjusted properly and is definitely sending the closed throttle signal back to the ECU. I can't recall which 2 pins to check continuity at, but you can look it up. When throttle is closed, the continuity should be closed circuit between the 2 correct pins, then when you slightly open the throttle you should hear a click and those 2 pins will then go open circuit.
If the ECU isn't getting the "closed throttle" signal, it won't go into idle mode. One easy thing to do is disconnect the throttle position sensor harness connector and see if the car's behaviour is the same or different. If it's the same, then you know the ECU probably isn't getting a signal telling it to go into idle mode.
If the car is LH2.4, the ECU also gets a speed signal from the speedometer board, telling it when it can go into fuel cut-off mode on deceleration and also when speed drops to zero to go into idle mode. Might also be possible to check this signal but not 100% sure. Start with the easy thing (TPS function and adjustment) first I'd say.
