A computer expert witness is a category of expert governed by the same procedural rules as any other — CPR Part 35 and Practice Direction 35 in civil, Criminal Procedure Rules Part 19 in criminal, the Family Procedure Rules Part 25 in family. The technical subject-matter does not create a separate discipline. The report has to declare the expert's overriding duty to the court, set out the material relied on, identify assumptions, and disclose the range of opinion where opinion is contested.
What the expert must do
- Address the questions posed by the letter of instruction — and only those questions.
- Set out the material relied on and any material considered but not relied on.
- State the assumptions the report proceeds on, and by whom they were provided.
- Distinguish clearly between fact, inference and opinion; identify the range of opinion where reasonable experts might differ.
- Concede the limits of the evidence and the limits of the examiner's own expertise, honestly.
- Include the mandatory statements of truth and understanding of duty to the court.
What the solicitor should provide
| Item | Why it matters |
|---|---|
| Written letter of instruction | Frames the questions, sets the standard (Part 35 / Part 19 / Part 25), and records the assumptions. |
| Pleadings and case summary | Lets the expert address the issues actually in dispute, not the ones the instructing solicitor thinks are in dispute. |
| Chronology and cast list | Names and dates the expert can rely on without asking, reducing time and cost. |
| Relevant evidence, in native form | The disk image, the mailbox export, the phone extraction — not printouts or screenshots of them. |
| Any prior expert reports in the matter | So the new report engages with, rather than duplicates, the existing analysis. |
| Privilege boundaries | A written statement of what the expert must not see, and how privileged material has been segregated. |
The letter of instruction that works
- 01State the procedural standard. 'Please prepare a report compliant with CPR Part 35 and Practice Direction 35' — and identify the court and deadline.
- 02Frame the questions. Two to six specific questions the tribunal needs answered, phrased so the answer can be yes, no, or 'the evidence does not support a conclusion either way'.
- 03Identify the material. List every source enclosed, with a manifest, and confirm the state in which it is provided.
- 04Set the assumptions. Say clearly which factual matters the expert should assume, and note that any change in assumption may change the opinion.
- 05Address privilege and single-joint status. State whether the expert is single joint, party-appointed, or a shadow expert, and how privileged material has been handled.
- 06Confirm scope of testimony. Whether attendance at hearing is anticipated, on what dates, and whether a joint statement with an opposing expert is likely.
Vague oral instructions produce vague reports. A tight, written letter of instruction is the single largest lever on report quality — and on the cost of getting there.
Where instructions typically go wrong
- Instructing on the device rather than on the question — 'examine the laptop' rather than 'establish whether documents X and Y were transferred to external storage between these dates'.
- Sending screenshots and printouts as the evidence, rather than the native files and images that produced them.
- Failing to identify the assumptions the expert is asked to make, leaving the report vulnerable on cross-examination.
- Instructing too late for meaningful analysis before the pre-trial deadline, forcing a supplemental report that opposing counsel will exploit.
Frequently asked questions
A single joint expert is instructed by all parties, owes duties to the court alone, and produces one report for the tribunal. A party-appointed expert is instructed by one side but owes the same overriding duty to the court. The choice is driven by the court's directions and the nature of the dispute.
No. The expert has to identify the material relied on, and Part 35 requires it. Assumptions can be made, but they must be stated on the face of the report.
The report is the expert's, not the solicitor's. It can be clarified, and errors of fact can be corrected, but the opinion cannot be redrafted. If a different view is needed, that is a shadow-expert instruction — a separate exercise, and one to take advice on.
Yes — including material considered and rejected. Withholding provided material creates a cross-examination point that will land.
Where opposing experts have produced conflicting reports, the court will often direct them to meet, without lawyers, and produce a joint statement recording agreements, disagreements, and reasons. It is a routine and important stage; the expert should be briefed on the topics in advance.