Shopping trolley damage to my car.?

On the 16th May 2011, my wife parked our car in a dedicated disabled bay ON THE QUEENS HIGHWAY (a normal road) in town, when a Tesco shopping trolley from the nearby store which does not have a car park, caused at least �250 worth of damage. Tesco's have been in touch to say they are not liable. How can this be? I could understand if we were in there car park, but we were parked on the road. This happened in a place where trolleys are often left by shoppers who get picked up by taxi. Any idea's on what to do next would be gratefully accepted.