Not done it for a while and your case differs somewhat as no one has died or claimed to have received an electrical shock. Reports I have done have been house fires, death from electrocution and people who have claimed to have received a shock in factories, the latter often chancers but in one instance they had a genuine claim. In your situation the fact things have 'rotted away' is irrelevant to the electrical installation so they are looking for an expert opinion to reinforce the claim over their incompetence. Personally I would compile a written report citing departures from BS7671, they don't want chapter and verse just facts and the possible consequences of these departures. You really need to know what they are expecting from you.