My bonnet wouldn't release last weekend, but it was the pull at the cable not popping the bonnet open so that the latch/hook could be moved across. Took both grills off and from what I could see there is little or no access to the actual cable release, but full access to the latch/hook.
Got in there with loads of WD40 and left for a while, few pulls on the cable and still nowt, tried pushing the bonnet down and heard/felt a click, pulled the release cable again and it popped open.
Once open I found the problem, near the washer bottle (mines a 2003 2.5) the release cable goes into a junction box that has another cable inside that goes to the release catch. The junction box is attached to the bodywork of the car. When you pull the release cable from inside the car, the cable pulls the second cable inside the junction box and then the opening catch. My junction box had come away from the bodywork therefore the second cable wasn't being pulled. Junction box reattached to the bodywork and the release lever feels like it's actually now doing something.
Might be worth checking on yours.