Thanks tiggytape. That's exactly what I was looking for.
This is part of their draft statement which I believe to be illegal:
"X is an independent, state funded school which has an agreed Published Admissions Number of 120 per year for admission into year 7 and shall admit up to this number each year. Where the school receives more applications for year 7 than it has places available the criteria listed below will apply.
Admissions Criteria
 Children with a statement of Special Educational Need (SEN) where X Senior School is named on the statement must, by law, be admitted to X Senior School.
Where multiple options have been listed for schools priority will be given to those placing the school as first choice. Only when all of the first choice places have been issued according to the listed over-subscription criteria will we issue places to those listing the school in either second or third place. After this requirement has been met, places will be allocated in line with the following objective over-subscription criteria in this order:
 Any request for the allocation of a place for children looked after or formerly looked after1 by the Local Authority and adopted children who were formerly looked after by the Local Authority will be admitted next.
 Children with a strong social or medical reason. This must be supported by written evidence from a professional such as a doctor or social worker involved in the case and must demonstrate why the school is the only one which the child can attend as a result of the social and/or medical need.
 Siblings of children who will be attending the school in the year of admission i.e. brothers or sisters including step, adopted or foster brothers or sisters living in the same family unit at the time when the child would be due to start at the school (note this criteria could not be applicable for the first round of applications forSenior School in 2015).
 Children whose names have been entered on the schools expression of interest prior to December 2013."
Thanks so much for your help