Window and door installation warranty

The company is not replying to my emails to arrange for a repair to be made to my rear door multilock system. I called them and the manager was trying to verbally change the T's and C's. I explained the terms and conditions but he was still not helpful or accepting of the terms and conditions given to me at the time of sale/ installation. I contacted citizens advice and then contacted the company by email due the difference in fact of the repair needed. Is there any other official bodies I can contact to back me up to show the window company I am not budging from the warranty terms.