How to Instruct a Warranty Claim Expert Witness

This guide is for UK solicitors and corporate counsel instructing forensic accountant expert witnesses in England and Wales M&A disputes. It assumes an English law sale and purchase agreement, CPR Part 35 where litigation applies, and ICAEW expert determination where the SPA provides for it.

Step 1: Identify the Dispute Type

Determine whether the dispute is a breach of warranty claim, completion accounts dispute, earn-out disagreement, or locked box leakage claim. Each mechanism requires a different expert approach and deliverables.

Step 2: Expert Determination or Litigation?

Review the SPA dispute resolution clause. Completion accounts disputes are typically referred to ICAEW expert determination. Warranty breach and earn-out conduct claims typically proceed to litigation or arbitration requiring CPR Part 35 expert reports.

Step 3: Single Expert or Party-Appointed?

Consider whether a single joint expert (SJE) is appropriate or whether each party should appoint its own expert. In High Court commercial litigation, party-appointed experts are standard.

Step 4: Accounting Standards Expertise Needed?

Identify whether the dispute involves IFRS, UK GAAP (FRS 102), or SPA-specific accounting basis issues. Ensure the expert has experience with the relevant standards and the specific accounting policies of the target company.

Step 5: W&I Insurance Involvement?

If W&I insurance is involved, the expert must address policy coverage, exclusions, and the insurer's likely scrutiny of expert evidence. Both buyer and insurer may appoint their own experts.

Step 6: Letter of Instruction

Draft instructions consistent with standard market value definitions. Inspired Education v Crombie [2025] demonstrates that improper instructions fatally undermine expert evidence. Identify the specific warranties, accounting basis, and valuation methodology.

Step 7: Documents to Provide

Provide the SPA (including schedules and disclosure letters), reference accounts, completion accounts, earn-out calculations, due diligence reports, management accounts, correspondence between the parties, and any prior expert reports. Include documents unfavourable to your client's position.