My DD2 (7mo) was after having her lunch followed by a Farley's rusk while seated in her highchair. The tray needed a clean so I detached it and, right in front of my very eyes my daughter fell from the seat and was suspended upside-down in mid-air by the harness. I managed to free her. After a lot of cuddles she was okay. I examined the harness and, to my horror, discovered the stitching had become undone on one side of it and broke away. Thankfully the stitching didn't come undone on the other side of the harness too (as could have happened while taking the full weight of DD2) or she would have fallen head first onto the hard ceramic tiles. As it is, she has a bruise on her thigh.
I'm so angry this happened. Seemingly it complies with BS5799. You imagine a child should have been safe in it, and this should never have happened. I only bought it in Argos a few months ago.