As far as I know, writing is purely teacher assessed - though there is a piece of short writing in the SPaG. DD2 got L6 SPaG and has been levelled as a 6 in writing by teacher assessment - mind you, writing is her 'thing', it's mostly effortless for her. When she tries, she is very, very good indeed. Based on that and on the fact that her teachers are a pair of very tough markers, I'm going along with what they say.