cascading combo boxes - I'm missing something simple here!

Hi Folks
I've been experimenting with cascading combo boxes and using this link as a reference.
http://www.access-programmers.co.uk/forums/showthread.php?t=223980&highlight=cascade

In the attached database (Frm_workcentre_only) , the first combobox (cbo_work_order_01) works fine. It just filters for the correct work orders for that work centre. However when I get to Work order 2, I want that combo box (which is based on Qry_cbo_workOrder_02 which is to exclude the choice made in cbo_work_order_01 does not exclude the choice.  For example if I choose 642642001 as my work centre number, I get three choices in WorkOrder1. If I choose 609120 in WorkOrder1, I want to have only the other two numbers available to me in Workorder2. Alas, I'm getting all three. As per the link above, this is what I have done...
In the AfterUpdate event of WorkOrder 1 I have entered the following code:
Private Sub cbo_work_order_01_AfterUpdate()
Me.cbo_work_order_02.Requery
Me.cbo_work_order_03 = Null
Me.cbo_work_order_04 = Null
Me.cbo_work_order_05 = Null
End Sub

In the AfterUpdate event of WorkOrder 2 I have entered the following code..
Private Sub cbo_work_order_02_AfterUpdate()
Me.cbo_work_order_03.Requery
Me.cbo_work_order_04 = Null
Me.cbo_work_order_05 = Null
End Sub

I keep feeling as though I am missing something really simple here and I  just need fresh eyes! If I can get this one working, I'll then have a way to get the other combo boxes working :-) thanks again
Workorder_database_vers_06.accdb