I think the old rules were that they could demand it for religious education teachers (and maybe they said that in a primary school, any form teacher would be teaching RE, but I'm not sure); from Sept 1st this year, they've allowed even for non-teaching posts:
As Accord is being launched new rules are being put in place to restrict even further the rights of staff in state funded religious schools. From 1 September 2008 it will be legal for voluntary controlled schools to reserve the headship for religious teachers and for voluntary aided schools to discriminate against non-teaching staff on the basis of their beliefs.
These changes have been in the pipeline since the passing of the Education and Inspections Act in 2006 so there was not much hope that we could change reverse the decision on the day of our launch! Nevertheless, we are determined that these appalling regulations will mark the high water mark of legalised religious discrimination in state funded schools.
http://www.accordcoalition.org.uk/index.php/2008/08/11/test-post/From my reading of these 2 links below, schools can reserve one fifth (rounding up) or their teaching posts (including the head, now) for their chosen religion, and are allowed to discriminate when appointing them, or deciding what they get paid.
School Standards and Framework Act 1998 (paragraphs 58, 59 and 60)
Education and Inspections Act 2006 (see explanation about Section 37)
Maybe a teaching union could tell you whether these 'committed Christian' ads are actually legal - you'd think that only a fifth of those from the Christina schools could say it, and no others. Yeah, I'd like all religious tests to disappear from state schools altogether.