I know nothing about this, but as always am willing to give my opinion
It seems to me that boys are often a bit behind girls in writing at this stage, and to tell you the truth, I have no idea what levels my boys were on when they went to GS - I know they were both a level 5 overall on English, but I couldn't tell you for the indiviual bits (maybe we weren't even told - shrugs - can you tell it's all behind me?), and I would strongly suspect that both were level 4 for writing. They are both doing fine. One is in year 10 and the English department would like him to do English A level; - he won't, as he still doesn't like writing very much.
I know that in Bucks they had to have levels 5s in all elements of English to be allowed to sit the 12 plus, however I know of at least one boys GS that waived this for writing once they were allowed to run their own 12 plus, so presumably that was an acknowledgement of the above.