The FENSA requirement is a nice little earner racket for installers. At the end of the day the main required specifications are met by the window manufacturer in the majority of cases all the installer does is fasten it to the opening, installs the glass and seals the frame/opening interface all of which any competent DIYer could do. Yet come to sell a house without a FENSA or other equivalent certificates gets convayencing lawyers having melt downs.

Richard