Consumers protection body

Dear xpats,
I replaced my windows in June last year. There is a problem with condensation in one of the windows. I've contacted the window company to get it fixed, they are saying that the problem is not with the windows or the installation of windows.
There is definitely a problem with it as the other windows on the same side of the wall sk t have this condensation problem.
I have 2 years guarantee but the windows company are trying to shy away from their responsibility.
I was wondering if someone could tell me what rights do I have as a consumer in this situation and who can I contact?
Many thanks for any help.