Installation

I have ordered new windows and paid a deposit some weeks ago. We were told a date for them to be installed but that was cancelled as they were over running on another job, We were told they would 100% fit them on 21st August, today. Waited until 10 a.m and then rang the office to be told they had problems with a conservatory that they were fitting and would not be coming. Promised again to come in a few days. How do we stand if we want to cancel.