It wouldn't work if anyone warned you. It dawns on the viewer at the same time she realises and that's what makes it so powerful I think. (Unless you saw it earlier.) I think it's one of their best and I love Sheridan Smith in anything.
I've seen it 4 or 5 times now and it gets me the same way every time. It's so raw. I found it so affecting, it's stayed with me. Agree with the PP though - it has such an impact because you don't know until she does.