You were missing an "EndIf" as well.
 Code:
;Set the string for the filename
$UIDINFO = "c:\Codes2b.csv"
;Check it exists
If Exist ($UIDINFO) = 0
  MessageBox ("Error1:Consultant ID File Does Not Exist."+ Chr(13)+
  "Please contact IT Support on 1234, as you will not be able to log in,"+Chr(13)+
  "You will now be logged off this system!.",
  "WARNING MESSAGE", 48, 300)
Else
  ;Check if the file is of a size greater than 0KB
  $UIDSIZE = GetFileSize($UIDINFO)
  If $UIDSIZE = 0
    MessageBox ("Error2:Consultant ID File Is To Small."+ Chr(13)+
    "Please contact IT Support on 1234, as you will not be able to log in,"+Chr(13)+
    "You will now be logged off this system!.",
    "WARNING MESSAGE", 48, 300)
    Sleep 10
  EndIf
  ;Open Up The UID File
  $rc = Open(1, $UIDINFO, 2)
  $line = ReadLine(1)
  ;Search for a match for the user
  While @ERROR = 0
    $line = Split($line, ",")
    If $line[0] = @USERID
      $uid = $line[1]
    EndIF
    $line = ReadLine(1)
  Loop
  ;Count the UID string and ensure its populated
  $uid_length = Len($UID)
  ;If statement that logs the user off if the UID is not of a suitable length
  If $uid_length = "0"
    MessageBox ("Error3:No matching Consultant Code Found."+ Chr(13)+
    "Please contact IT Support on 1234, as you will not be able to log in,"+Chr(13)+
    "You will now be logged off this system!.",
    "WARNING MESSAGE", 48, 300)
    Sleep 10
  Else
    Set "CONS_CODE=$UID"
    ? "Found name with UID: " + $uid
  EndIf
EndIf


Please post a line or two of the contents of the CSV file.