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The law itself is fairly ambiguous which leaves a lot of room for interpretation and the airlines are good at exploiting every potential loophole. That means that all the contentious details are up to the courts to sort out on a one-case-at-a-time basis. The legal documents backing this up are court decisions.
Here is a list court cases for some of the major disputes: https://flightdelaypay.com/leading-cases/
Your specific case is covered in Case 14. It lists the judge and the court where the ruling was issued. Each court will keep records, but it will vary from court to court how you can get your hands on them and you probably will need to contact them directly.
Is there any legal document backing this claim up?
Yes. The legal document is the court ruling by Deputy District Judge Benson made in the County Court of Macclesfield on 16th September 2013.
But if you have the legal document: then what? The airline will quiet likely still refuse to pay. Your options are typically