The bonnet release system on the Rover 75 is made up of three cables. The first cable runs from the release handle to a plastic junction box situated on the off-side front inner wing panel by the windscreen wash bottler filler neck. Two cables run from this junction box to the left and right bonnet latches. Chances are that the primary cable has stretched because the two secondary cables have become partially seized. One option is to have a friend push down on the bonnet to see if it will relieve enough pressure to allow the mechanism to properly function. Another option involves manually manipulating the cables. To do this, turn the steering as far to the left as you can and reach inside the small inspection hatch at the top of the wheelarch liner (you'll need a torch so you can see what you're doing - and you can push the neck of the washer bottle up and off to give yourself more room). Unclip the cable junction box from the inner wing and pull it through the small hatch. Tug on the shuttle inside the junction box (plastic thing that the three cable ends go into) to pull the two cables to the latches (you should be able to do this without resorting to pliers). If it works, one latch will probably pop open first then the other, allowing you to release the stuck bonnet on your Rover.