@DENIS80 From the brief extract from the complaints policy I'm assuming this is the full policy. As it is a generic policy applying to all CfBT academies and does not name any individual school this link does not identify the school you are complaining to.
https://www.cfbtschoolstrust.com/attachments/download.asp?file=64&type=pdf
[Slight digression, but while searching for the policy I discovered
here that CfBT is changing its name from September to Anthem Schools Trust]
I know you didn't ask for comments on your complaint itself, but I'm going to give my view anyway

(I am an experienced governor on complaints panels and a National Leader of Governance, but I have no connection or involvement with CfBT).
IMO the school is acting correctly within its complaints policy and ESFA would be unlikely to find significant error. It may have exceeded some timescales, I can't be sure from your posts, but in essence I think they are entitled to refuse to consider a complaint from you.
Firstly I think the policy only allows complaints from parents - it says it is for
"...complaints raised by current parents or legal guardians and others. It may, at the school’s discretion, apply to a parent whose child has recently left the school."
IMO "others" means people with a similar status to parents - other people who have parental responsibility, such as grandparents a child lives with. You aren't in that category. You personally have no parental responsibility or family relationship to any pupil at the school so aren't eligible to bring a complaint. The law on academy complaints policies does not require them to allow complaints from people who aren't parents.
But let's suppose the school accepted your argument that you aren't bringing the complaint in your own right but on behalf of/as agent for someone who is a current parent. But you refuse to tell the school who this parent is (and presumably therefore you have no written evidence to give to the school to show that you are acting on behalf of a parent). So as far as the school is concerned this an anonymous complaint - they don't know the name of the parent or the pupil. It's the same as if the parents themselves had sent in an anonymous/unsigned letter of complaint through the post.
Page 8 of the Policy says:
"Anonymous complaints
The school will not investigate anonymous complaints under the procedure in this Concerns and Complaints Policy. Anonymous complaints will be referred to the Headteacher who will decide what, if any, action should be taken."
It appears the school has followed its policy on Anonymous Complaints and the action they have taken is to tell you that they will not consider it further unless the parents themselves raise it as a complaint.
And IMO there are other reasons, nothing to do with the policy, why the school is correct to refuse to discuss this with you. From the school's point of view (I am not saying I necessarily agree with them) you are a total stranger who has walked in off the street (so's to speak) claiming to be acting for an anonymous parent, who you won't name and don't have any written letter of authority from, asking the school to discuss with you other people's children who you do name but have no connection with. In those circumstances I'd be surprised if any school was willing to discuss the matter with you. It would potentially be breaking data protection law and your request could raise a safeguarding concern.
However, if you are still convinced the school is in the wrong then the place to complain about them is the link to the ESFA I gave you earlier, and let us know how you get on!
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