well i know that cats are standardised to a national cohort and so is the kent test - many 11 plus areas use a local chort to standardise - so in this respect they are more similar than some.
I appealed last year to a school that used a cut of score - my dd passed but was was a few points from it. I based my appeal round why that particular school would be suitable for my dd - specialisms - ease of transport - her first choice etc but i did also include 2 letters from her school. One was a general recommendation from the head but the other was from one of her year 6 teachers who confirmed she was already at level 5 at the time of my appeal. I wasnt sure weather to include it as my appeal was basically an over-sub one but i decided after chating with people to include it. I was pleased i did as one of the panel asked me how i thought my dd would cope as she hadnt met the cut off score! I referred them to this letter and the school rep actually did say at this point to the panel member that mine was not a non-qual appeal as my dd passed. This letter was only a very small part of my appeal and was just put in as an appendix without much written about it by me.
Check out the feedback area in the appeals forum as you can see what others have done there.
I think generally if there was a reason why your dc did not score higher on the day you can expand more about regarding his ability.
I am sure someone else will be slong with more advice and also help regarding the NVR score.