Andrew Spence
Check to see if FENCA registered, if they fit window they are likely to be registered with them, they will have a complaints procedure. If not FENSA then local building inspector is next best bet followed by trading standards. Have a look to see if windows shut fully with no gaps, and check around edge it could be that the edges have not been filled when the window was put in and if so is an easy fix using spray foam, moulding and mortar.