Anyway, you are right that a small sample does not equate to evidence, but there is plenty of evidence on this.
It is difficult to judge the veracity of that study as they have only published what is effectively a press release (can't even really call it an abstract...). However the London Diocesan Board of EduÂcation (CofE) have disputed the findings as far as their schools go.
"In the London diocese there are 149 schools, where 11,399 pupils (23 per cent) receive free school meals, and for 15,878 pupils (37.5 per cent), English is an additional language.
The vast majority of our schools, particularly our primary schools, serve their local areas, and thus represent the social backgrounds in which they are set.
The intake of Anglican compreÂhenÂsives is more diverse, because they may be the only church secondÂary in a larger area. For example, at St Marylebone C of E School for girls, one of the most sought-after in the country, in an exclusive area of LonÂdon, 56 per cent of pupils have a home language other than English, and 44 per cent receive free school meals. "
They had also look in depth at Camden in response to claims by Frank Dobson about faith schools in his constituency:-
"There are 13 Church of England primary schools in Camden, but no Church of England secondaries (and no C of E secondary for the nine C of E primary schools in Islington or the five C of E primaries in Brent).
The average percentage of pupils in the Camden C of E primary schools from ethnic-minority backgrounds is 53 per cent. (The average for all Camden primary schools is 39 per cent.)
The average percentage of pupils in those schools from British Asian and predominantly Muslim backgrounds is 20 per cent; in three schools the proportions are above 40 per cent. (The average for all Camden primary schools is 17 per cent.)
The average percentage of pupils with English as an additional language is 48 per cent. (The average for all Camden primary schools is 48 per cent.)
The average percentage of pupils with Special Education Needs is 32 per cent. (The average percentage for all Camden primary schools is 26 per cent.) "
From this & stuff I've looked at before I suspect that RC schools are typically more of an issue than CofE schools.
Although I'm sure someone will find an example of one that does, CofE schools typically only ask about parental worship attendance, not baptism before x months old (or baptism at all in fact), not what voluntary roles you do in church, etc - this type of "points" system is a lot more common in popular RC schools.