Talked to our insurer. It was all a mistake. As I thought, we should not be charged anything for insuring our hire car, as although it's bigger than the policy covers, it was not provided at our request but because their hire company didn't have the right size available. We have therefore had our money refunded. Hooray! And thanks Natalia, for sorting it all out. Unlike her colleagues on Saturday, she knew exactly what she was doing.