Isn't this something you would want to do with XPath? Or perhaps I am misunderstanding your intention...perhaps your idea is to avoid parsing the file again (at the later date) and just do seeks to known offsets.

[Judi Thomson]   

    Yes, that's it exactly.  I don't want to have to parse the files again because they're potentially very large.  I want to use a known offset to get to the relevant record in the file, pull that out and *then* I'll use xpath to get at the rest of it.

Judi