Engagement letter
Scrutiny work goes wrong most often over scope: the trustees expected advice, the examiner expected records, and neither said so in advance.
The letter should state the scope of work, the respective responsibilities, the fee basis, and the timetable. For an independent examination it should also confirm that the work will follow the Directions.
Read it rather than signing it — particularly the section on what the charity must supply and by when. See appointing an examiner.