This is not entirely true Lee, none of them are oem manufacturered as far as I know, they use several companies to produce for them, but replacements are usually always manufacturer specification for safety sake cause most are changed through insurance companies anyway so have to be covered otherwise has legal ramifications, so doubt they're cheap copies or **** made, had several screens & they have always been as good as last.
The windscreen companies cant afford to sell **** because of the work they would lose & also bottom line safety guys, legally they'd be seriously fecked if the screens were poor, with the new bonnets mods for euro crash tests etc you can imagine the glass requirements aswell.