Doh - Of course!! In my job I'm so used to reading text data in one form and writing it out to a new file in a usually entirely different form that it didn't even occur to me to just alter the existing file.

So I should have opened the file with the "+<" mode for read/write, seek to position 148 and write the byte there. (I would still need to copy the file first as I need to keep the original.)

Talk about closed thinking - I can't believe I overlooked that. Thanks adrianh and ++ (once I get some more votes!).

Cheers!