If you have already taken delivery you need to send a letter by recorded delivery to the dealer principal of the Evans Halshaw dealership stating that you took delivery under protest as the car was incomplete and therefore "not of satisfactory quality" because it came with only one key. Then state you give the dealership the opportunity to replace the missing key or take the car back and give you your money back. You'll soon get an offer of a second key, though only a remote key if the standard spec of the car is two remotes. Most new cars come with one remote and one standard key. The possibility that someone else still has a key to your car cannot be resolved other than by changing all the locks, which is not a "reasonable" thing to ask.