Part 35 of the Civil Procedure Rules governs the use of expert evidence in UK civil proceedings. For slip-claim litigation, where pendulum data and surface assessment are typically central, Part 35 compliance is not a formality — it determines whether the expert's report and evidence are admissible and how much weight the court gives them.
CPR 35.3 establishes that the expert's overriding duty is to the court — not to the party who instructed and pays the expert. This is the foundation of expert evidence under English law. An expert who advocates for the instructing party, conceals adverse findings, or shapes opinion to fit the instructing party's case is in breach of Part 35 and produces evidence that collapses on cross-examination.
For pendulum experts, this means: the data is the data. If the test result favours one side clearly, the report says so — even where both sides hoped for a more nuanced outcome.
CPR Practice Direction 35 specifies what every expert report must contain:
Reports without these elements are exposed to admissibility challenge.
Where there is a range of legitimate opinion on a matter, the report must summarise the range and give reasons for the expert's own opinion within it. For pendulum testing, this is sometimes relevant where the test result is in the moderate band (PTV 25–35) and reasonable experts might differ on whether the surface presents an actionable risk in the specific context. The Part 35-compliant approach is to set out the considerations either way and explain the expert's reasoning.
For lower-value or single-issue claims, the parties may agree to instruct a single joint expert under CPR 35.7. Both parties share the cost (usually equally), agree the letter of instruction, and receive the report simultaneously. The expert's duty under Part 35 is unchanged — the duty is to the court. See our joint expert scenario page.
Each party may put written questions to the other party's expert, intended to clarify the report's content. The expert's answers are treated as part of the report. This is a useful early step that often resolves apparent disagreements without requiring an experts' meeting.
Where each party has appointed their own expert, the court may direct that the experts meet to identify points of agreement and disagreement and produce a joint statement. This is one of the most useful procedural mechanisms in CPR 35 — narrowing the technical issues before trial and often producing settlements when the experts substantially agree.
Where matters proceed to trial, the expert may be required to give oral evidence and undergo cross-examination. The strength of the report is the foundation, but the strength of the expert in the witness box matters too. Methodology that withstood internal review must also withstand opposing counsel's cross-examination — which is one of the reasons UKAS-accredited methodology and clear, neutral writing pay dividends in litigation.
The Part 35 duty applies regardless of which party instructs. Where we are instructed by a defendant and the test data shows non-compliance, the report says so. Where we are instructed by a claimant and the test data does not support the claim, the report says so. Over time, this is what makes a report defensible — the expert who only tells the instructing party what they want to hear has no enduring credibility.
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