Lots of smash bang wallop solutions on these forums but I've yet to find anyone explaining WHY this problem occurs. This post applies to pre-2004 models; it's probably not relevant to newer wheels. A short steel rotating shaft connects the lock to the bonnet release latch. At the latch end shaft diameter reduces. A circlip is inserted here to ensure correct pressure is maintained at the lock end. If this clip is missing or worn too thin the inserted key will not engage. With luck and perseverence putting upward pressure on the key and trying to pull the lock towards you will bring sudden clicking joy. If not, forget all thoughts of grill smashing or lock drilling. Underneath the car is a thin access hatch between bumper and radiator. This is secured by four plastic pop rivets which can be unscrewed. Very little of the latch mechanism can be seen from underneath but poke a stick or long screwdriver up through the gap you've created by opening the hatch. Again with luck and perseverence, by tapping or scraping your implement close to the lock/latch while another person keeps working the key you will hear that happy release sound. Once the bonnet is open you will see exactly what the problem is. By gripping the connecting shaft with long-nose pliers and putting pressure towards the lock the key will work exactly as designed. If you're familiar with circlips pop a new one on and that's it. If not, wrap any piece of strong wire around the latch end and nip it tight with pliers. Garage cost: at least £100. Your cost: 30 mins of sweat rewarded by elation.