Is this an Optional Year 5 NFER paper? If it is, then surely there is only one paper that can be set so that all the kids (and parents) are able to tell how their child is doing. It does seem slightly strange that the head teacher is suggesting that they had a range of year 5 papers to choose from and picked a difficult one.... because as you say, it makes a potential appeal so much more difficult.....
I was always under the impression that there was only one optional NFER paper per year group with a marking schedule so that if you were using it as evidence at a later date it was comparable - but maybe I've been misguided all this time!!
Hopefully someone with more knowledge will be along shortly to advise.