I decided upon a value list which contains "Yes";"No", the query is then based on
SELECT Tbl_Mytable.PersonID, Tbl_Mytable.name, Tbl_Mytable.Grade_ID, Tbl_Mytable.Leader, Tbl_Mytable.[COR Int]
FROM Tbl_Mytable
WHERE (((Tbl_Mytable.Leader)="yes"));
I used the value list, rather than the tick...