It's not so much that they refuse to disclose the methodology, it's that there is no methodology. The rules are (I suspect intentionally) vague and the degree to which any given factor (duration of contract, working practices etc.) applies, seems to vary from case to case.

This creates an environment of great uncertainty for freelancers, but also one where HMRC can never be sure themselves, if they're properly pursuing those accused of being in breach.

They seem to regularly lose many of these cases on appeal when a (presumably) fair minded judge is asked to look at the facts.