You'll see "Part 35 report" or "Part 35 compliant" mentioned constantly around expert witness work. It's worth being precise about what it actually means.
Part 35 of the Civil Procedure Rules sets the ground rules for expert evidence in English civil litigation: the expert's overriding duty is to the court, not to whoever is paying the fee — and the report has to say so in writing.
That single duty is what separates a genuine expert witness report from an advocate's opinion dressed up as one — and it's the difference that gets tested first under cross-examination.
