According to EU Regulation 261/2004, "If an operating air carrier places a passenger in a class lower than that for which the ticket was purchased, it shall within seven days, by the means provided for in Article 7(3), reimburse [some money]."

As the business class experience was different from what was advertised, you could maybe argue that there were three classes: economy class, an unadvertised class in which you were placed and business class which you had paid for. As you were moved from business class to the unadvertised class, you would argue that you are entitled to downgrade reimbusement. However, it would just be 30% of the ticket cost excluding taxes and fees.