I'm sorry. I'm not exactly clear on what you're trying to accomplish. It sounds like you're trying to prevent a duplicate SiteCode from being entered via a ComboBox.

If so try doing something like this in the Valid method (I haven't tested this):

llRetVal = .T.
lnOrigArea = Select()
lcValue = This.DisplayValue

Select (SiteCodeTable)
Locate for SiteCode = lcValue
If Found()
  =MessageBox("Message to user")
  llRetVal = .F.
Endif 
Select (lnOrigArea)
Return llRetVal

As for the other things, again I'm not really sure what's going on, but some suggestions...

>1. Can type a value only once. i.e. assume values are A and B, then if I type an A I cannot type a B in its place but I can only change it via the drop down list.

Is the ControlSource only 1 character long? If so you might have to highlight and delete the value before typing another one.

>2. When a new value is type in instead of being set by the drop down list its not always validate the new value, i.e. if records exist for A and B it does not display the message that data already esists and moves to the next field as if value is ok. Save does produces an error message but I want to avoid that.

Instead of using the the combo's .value property try using the .displayvalue property.

Aloha,

James