No, unfortunately I doubt you'd have any case against the insurance company. Each company would have a large policy stating that if any part of your vehicle is deemed to be illegal than coverage will be forefeited. It would probably also state that by taking out the policy you're submitting and ensuring that your car and its modifications are completely legal. The fact that they stated that they would cover your car would be irrelevant as it is subject to your car being completely legal. The onus is on you to make sure your car is legal and hence covered under the policy, it is not the responsibility of the insurance company to ensure that all your modifications listed are legal, otherwise they would be forever running around checking laws, which in many cases aren't very clear cut. Understandibly this is a very bad situation for us, however, if your car is legal and they dont cover you, there is a totally different story at hand. The best you can do is try to ensure all of your modifcations are legal and keep your insurance company up to date as to what is done to your car. Also ring and clarify with your insurance company as to what is and is not covered under your policy....they are, in most cases, obligated to explain to you what you can do without breaching your policy.