OK,
With my subclass of Combo, I added an array called aPicklist in the class designer so that it was always available for all child classes. I also added a property called PKValue. The code for the InteractiveChange event in the 1st Child was

if This.ListIndex > 0
   This.PKValue = This.PickList[This.ListIndex,1]
endif

For any subclass from now on, during its Init, create another array (from for example) a Select statement like

SELECT PKCUST,cName,cSurname from CUST where cSurname = "COX" into array aArray

and the rest would be

 ACOPY(aArray,This.aPickList)
for m.i = 1 to alen(aArray,1)
   This.AddItem(This.aPickList[m.i,2] + This.aPickList[m.i,3],m.i,1)
endfor
&& if you want to make the first item the default
This.Selected(1) = .t.
THis.PKValue = This.aPicklist[This.ListIndex,1]
DoDefault()

Now, everytime the user changes the seletion in the Combo, the real PK value is always available with This.PKValue

Hope this helps

Pete