OK,

To rule out any strangeness that may be being caused by the DataTable I'm
trying to update, I'm creating my own just for this test:

So:

private void CreateDataTable()
{
 DataTable dt = new DataTable();
 dt.Columns.Add("PartNo",Type.GetType("System.String"));
 dt.Rows.Add(new string[]{"AAA"});
 dt.Rows.Add(new string[]{"BB"});
 dt.Rows.Add(new string[]{"C"});
 UpdateDataTable(dt); // This is the method that is in my OP.
}

This still gives exactly the same problem, ie the data written into the
database is:
AAA
BBA
CBA

To (hopefully) reproduce the problem, you need a windows form with a button
that calls the above method,
plus the earlier method:

Private void UpdateDataTable(DataTable myDT)
{
 OleDbCommand comm = new OleDbCommand();
 comm.CommandText = "AddComponentsForJobT";
 comm.CommandType = CommandType.StoredProcedure;
 OleDbConnection conn = new OleDbConnection(CONNECTION_STRING);
 conn.Open();
 comm.Connection = conn;
 comm.Parameters.Add("pPartNo",OleDbType.VarChar,10,"PartNo").SourceVersion
=  DataRowVersion.Current;
 da.InsertCommand = comm;
 da.Update(myDT);
}

plus an Access database containing a one table called 'PropJobParts' with a
single(text) field called 'PartNo'
And one update query called AddComponentsForJobT:

PARAMETERS pPartNo Text ( 255 );
INSERT INTO PropJobParts ( PartNo )
SELECT [pPartNo] AS Expr2;

If anyone has the time and the inclination to check this out, I would be
most grateful, I'm sure that I'm probably just missing somthing daft.

Thanks,

Chris.