Iberia refusing to refund

Iberia changed one of our return flights without informing us at all. We then incurred extra costs because we had to stay in Europe longer and now they are refusing a refund. First they delay and take a very long time to respond (hoping I will go away) and then claim they did inform us of the change. Three separate parties with different reservation numbers, three different telephone numbers and three different e-mail addresses and no-one received any notification.