You don't really show a picture of your "weir" box, I'm presuming that it's what us Yanks call an overflow

, but if it's glass, and siliconed into place, I think the only way to do i would be to drain the tank, remove it and have it cut. I don't see any other way to do it safely. You MIFHT get away with using a glass cutter & pliers to break it at the score mark, but I'd be awfully nervous that the whole thing might break, and you'd really have a mess.