SQL & PL/SQL :: Updating A File With UTL_FILE

I have an application that creates files using the utl_file package. It works fine but one of the lines in the file should hold the number of bytes in the file (on a line formatted like 'FileSize: 2104'). Unfortunately this line is not the last line of the file and the lines that follow it are variable in length.

My approach therefore is as follows:
1. Write the 'FileSize: ' line during file creation.
2. Write the remaining lines of data to the file (but don't close it).
3. Use utl_fgetattr to find the file size.
4. Go back and find the 'FileSize' line I need to update, using get_line to read it into the buffer.
5. Append the filesize (plus the number of characters that the variable I use to store the filesize value) to the buffer string.
6. Write the line back to the file using put_line.

However I keep getting invalid file operation errors however I try to re-access the file...

PROCEDURE setUtlFileSize (pFileName IN VARCHAR2,
pFileHandle IN utl_file.file_type) IS
vbFileExists BOOLEAN;
viFileLen NUMBER;
viFileBlockSize NUMBER;
vsFileRecord VARCHAR2(2000);
[code].......

This gives me the following output.... (for two files)... currently I am using some generic exception handling just to show me the error.

File Length: 2106
File Position: 0
-29283,ORA-29283: invalid file operation
File Length: 497
File Position: 0
-29283,ORA-29283: invalid file operation
Process exited.

Two things here concern me: The File Position suggests that the current offset position is set to the start of the file... but since I had not closed or performed any other operation since the last put_line and fflush (which are used to add lines to the file) I had expected that the offset position would be the same as the file length?

Secondly: Even if the position had been reset to the start of the file I don't understand why the get_line gives me the oracle error.