Decrease serial number if matching file found

Hi Everyone, Any help with any part of this issue would be a great help. I have posted this a couple of times before. You are probably getting sick of seeing this but I hoped someone may be able to help, I'm not giving up on this issue.
http://www.excelforum.com/excel-prog...ml#post3530142

So to the brain achingly annoying issue. What I'm trying to achieve is;
1. When a new document is opened from my template a unique serial number (in format "00000") is generated (currently using txt file)
2. Once the number is generated a search is preformed to see if a file exists saved with the same serial number as a name.
3. If a file exists then the txt files stored value is decreased by 1 and that is displayed in "K5".
4. If the search returns a 0 then the original serial number is displayed.

Function GetSerialNumber(Optional FilePath As String, Optional FileName As String) As String

  Dim DefaultName As String
  Dim DefaultPath As String
  Dim FSO As Object
  Dim SeqNum As Variant
  Dim TxtFile As Object
  Dim TargetCell As Object
  
    DefaultPath = "C:\Documents and Settings\bethan\My Documents\Scripting\Delivery Note Job"
    DefaultName = "Invoice"
    
      FilePath = IIf(FilePath = "", DefaultPath, FilePath)
      FilePath = IIf(Right(FilePath, 1) <> "\", FilePath & "\", FilePath)
    
      FileName = IIf(FileName = "", DefaultName, FileName)
      FileName = FilePath & IIf(InStr(1, FileName, ".") = 0, FileName & ".txt", FileName)
    
      Set FSO = CreateObject("Scripting.FileSystemObject")
    
       'Open the file for Reading and Create the file if it doesn't exists
        Set TxtFile = FSO.OpenTextFile(FileName, 1, True, 0)
      
       'Read the serial number
        If Not TxtFile.AtEndOfStream Then SeqNum = TxtFile.ReadLine
        TxtFile.Close
      
       'Update the serial number
        Set TxtFile = FSO.OpenTextFile(FileName, 2, False, 0)
        SeqNum = Format(IIf(SeqNum = "", "1", Val(SeqNum) + 1), "000000")
        TxtFile.WriteLine SeqNum
      
   TxtFile.Close
   GetSerialNumber = SeqNum
   
     
     With ThisWorkbook.Sheets
            With Cells(7, 11)
                If IsEmpty(.Value) Then
                    .Value = Date
                    .NumberFormat = "dd mmm yyyy"
                End If
                End With
                 
   Set FSO = Nothing
   Set TxtFile = Nothing
   End With
   End Function
   
Function MinusOne(Optional FilePath As String, Optional FileName As String) As String

  Dim DefaultName As String
  Dim DefaultPath As String
  Dim FSO As Object
  Dim SeqNum As Variant
  Dim TxtFile As Object
  Dim TargetCell As Object
  
    DefaultPath = "C:\Documents and Settings\bethan\My Documents\Scripting\Delivery Note Job"
    DefaultName = "Invoice"
    
      FilePath = IIf(FilePath = "", DefaultPath, FilePath)
      FilePath = IIf(Right(FilePath, 1) <> "\", FilePath & "\", FilePath)
    
      FileName = IIf(FileName = "", DefaultName, FileName)
      FileName = FilePath & IIf(InStr(1, FileName, ".") = 0, FileName & ".txt", FileName)
    
      Set FSO = CreateObject("Scripting.FileSystemObject")
    
       'Open the file for Reading and Create the file if it doesn't exists
        Set TxtFile = FSO.OpenTextFile(FileName, 1, True, 0)
      
       'Read the serial number
        If Not TxtFile.AtEndOfStream Then SeqNum = TxtFile.ReadLine
        TxtFile.Close
      
       'Update the serial number
        Set TxtFile = FSO.OpenTextFile(FileName, 2, False, 0)
        SeqNum = Format(IIf(SeqNum = "", "1", Val(SeqNum) - 1), "000000")
        TxtFile.WriteLine SeqNum
      
   TxtFile.Close
   MinusOne = SeqNum
   
     
     With ThisWorkbook.Sheets
            With Cells(7, 11)
                If IsEmpty(.Value) Then
                    .Value = Date
                    .NumberFormat = "dd mmm yyyy"
                End If
                End With
                 
   Set FSO = Nothing
   Set TxtFile = Nothing
   End With
   End Function
   
  
Function CheckIFVoid(Optional FilePath As String, Optional FileName As String) As String

Dim MyFileCount As Integer
Dim DefaultPath As String
Dim TargetCell As Object
Dim Execute As Variant

   DefaultPath = "C:\Documents and Settings\bethan\My Documents\Scripting\Delivery Note Job"
    Set TargetCell = Sheets(1).Range("K5")
    
With Application.FileSearch
.NewSearch
.LookIn = DefaultPath
.FileName = TargetCell

 MyFileCount = 0
  If .Execute() > 0 Then
  Sheets(1).Range("K5") = MinusOne()
  End If
 If .Execute() = 0 Then
 MsgBox ("nothing found")
 End If
  End With
End Function

 Sub Auto_Open()
If Cells(5, 11).Value = "" Then
  Sheets(1).Range("K5") = GetSerialNumber()
  End If
If Cells(5, 11).Value >= 0 Then
Sheets(1).Range("K5") = CheckIFVoid
  
  End If
End Sub

Currently i don't belive the search function is working. So any help with that would be greatly appreciated and oddly once the search fails to run properly the original serial number dissapears.

Thanks Everyone.