How do I see the group structure as it was on a past date, e.g. at the year end?

Company Structure → Chart → Structure as at: pick a date and the chart redraws from the holdings in force on that date — the position at a year-end, before a transfer, or on the day the auditor asks about. Today clears it. The date is kept in the address, so a refresh or a shared link shows the same date.