Prefill SaveAs Dialog Box

I have looked at all the threads for this and tried all the codes to no avail. I currently have this code:

Public Sub SaveAsG4()
Dim SavFil As String
Call CopyToG4
SavFil = Sheet1.Range("G4").Value
Application.Dialogs(xlDialogSaveAs).Show (SavFil)
End Sub

The SaveAs Dialog box appears - but the filename doesnt appear in the filename box.
This is doing my nut in.
Whats really annoying - is that it used to work before, and now it just doesnt want to play.....

Any Ideas Peeps......

Craig