Hi, welcome to the site. My name is Max and I will help you with your question.
Do they allow appointments to be rescheduled in their terms and conditions?
It not the window installer who has let us down but the window manufacturer
Your contract is with the installer though, right? The manufacturer made the windows for the installer, who is then to instal them for you? Or, did you contact the manufacturer directly to order the windows?
Hello, are you still there?
You would have the right to obtain compensation if they did not have the right to change the appointment time. However, this compensation isn't likely to be large, it's probably going to be something like £100 for the day you lost from your holiday.
No the manufacturer has not made the windows as yet. The installer has said that the delay is down to the manufacturer letting him down by not producing the windows to install as promised
Then your issue is completely with the installer. It's down to him to ensure the windows come in time for the appointment, the manufacturers is his problem.
If you were to sue anybody then, it would be the installer, for loss of the enjoyment you would get from your holiday due to his breach of contract.