this is the threshold and is part of the whole casing.
apart from maybe gluing and securing a thin metal plate in the recess across either side of the split there is not much you can do.
you can buy a small gap filler for upvc which acts as an adhesive and also lengths of arc (plastic strips) which come in various widths, 25,45,65,70,95mm etc. which can be stuck on to hide any unsightly cracks these come in 5mtr lengths.
knob-free, March 2009