I have actually sorted this on my son's car last summer. You'll need a long, long flat-bladed screwdriver. Kneel down in front of the car (and prey !) facing the centre of it and crouch right down. Looking up through the front grille you will be able to see the lever which operates the release - the assembley is mounted on the underside of the bonnet striker plate. A torch might be helpfull to see. Carefully poke the screwdriver up through the grille slots to move the lever the same way as pulling the cable would. It will help if somebody pushes down lightly on the front of the bonnet while you do this and you are likely to find the mechanism stiff. Once in, don't just fix the cable but actually lubricate everything too. What often happens is that the bonnet release stiffens up due to lack of lubrication (sevicing not done properly). Sometimes this leads to the release cable snapping, but often a different problem. The inside of the release cable has a fixed nipple which locates in the release lever. The outside of the cable is crudely clamped to the striker plate with just a one-screw 'P' clamp. If everything gets too stiff, the clamp can fail to hold the cable outer fast and it can slip - therefore no force applied to release the bonnet. Look for tell-tale marks on the cable outer to see if it has moved in the clamp. Worth having a Haynes manual to hand so you can see the arrangement.