Hi Flicka feel I should post a reply for you !
My son is 11 and in yr 6 and I agree it's difficult to assess yourself their level when it comes to writing essays etc.....there are often no right or wrong answers!
I don't think my sons essays will be much help as he will usually just write 1 paragraph with practically no punctuation, although apparently the content and vocab he uses is good. Have you thought about contacting The Home Education Advisory Service or something similar ? They may be able to give you info as to what criteria your son should fulfil when writing.....
I can share with you a recent worksheet my son received from school for a piece of writing :
Your work must contain the following to be at least level 4
A range of sentence lengths including complex sentences
A range of sentence openers e.g Suddenly, A little while later , Terrified , he ran from the
Interesting vocabulary chosen to entertain and excite your reader
Capital letters, fullstops and other basic vocab
Full of detail
This was homework for all children, so those just getting to level 4 up to those who are already level 5 so is prob a bit basic ! You will need to expand !