Geoff,

Wrong plastic. You need to try polycarbonate (Lexan) if you want to use something that is quite break resistant. I watched with fascination as a fireman, with an axe, tried to whack his way through a polycarbonate glazed window to gain access to the second floor window at a building with an apparently small but smokey fire. (I talked to the building owner later to find out what the wonder window was made of) The fireman finally got through, but his initial attempts were quite fruitless.

Apparently vandals, or spurned swains, liked throwing stones at that window and something had to be tried to keep the glass replacement costs down.