A clear timeline can make the legal assessment more reliable. Note when you noticed the accident, when you started helping, what you did, when the danger changed and when organized rescue personnel took over. Distinguish what you saw yourself from information someone else gave you.
Keep medical records, rescue-service notes, names and contact details of witnesses, photographs and any messages about the incident. Record whether you were on duty or covered by another statutory scheme. These are practical ways to preserve relevant facts, not a statutory checklist or filing deadline.