I must admit, I didn't save the whole thing before trying to split. I should have done that as matter of good practice. I shall give that a go.
Thanks for reply.
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The saving of the file periodically seemed to do the trick. Curiously, when I reloaded the file after splitting it. The main file didn't retain the bookmark information. I'm not sure if the process of bookmarks but I would have thought it would have been able to store it as a cue file at least.
Possibly, saving the split file as the bookmark name might be nice as well. I'm guessing that's a luxury on this platform. However, job done.