Is there a particular reason to be using the Clipboard?

If not, try this instead:
VB Code:
  1. Private Sub cmdDoIT_Click()
  2.    
  3.     If (txtLogNo.Value = "1") And (chkTaken.Value = True) Then
  4.         Sheets("Brochures").Range("F4").Value = txtQuantity.Value
  5.     End If
  6. ...
  7. End Sub