Have now resolved the issues (with the help from your support team).
I think the underlying issue was with the post_texts file - i had rogue CR/LF characters which were causing the import to fail.
I used my local install of my legacy BB to extract and reformat the CSV files to make sure they fully complied with the yaml formats. This included running corrective scripts against the raw data to remove rogue characters.
Its been an interesting learning experience <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />
