Repair Company Complaint

I have been waiting four months for a repair company to fit a spare part that they have ordered to repair my tumble drier. Every time I call, they tell me that the part is not here yet, and I must wait another week. The tumble drier is still under warranty so I can't use an alternative company. Also, I paid for the job the first time the service engineer called at my house. Is there a Belgian Organisation such as Trading Standards that I can refer the company to? Thanks!