I’ve found a workaround for this so thought I’d update, in case anyone else tries to do the same thing. Basically, if I add my sort order field to the columns well as the FIRST field, I can then apply the sorting to it and it seems to take precedence over the natural alphabetised sort order of the actual column names. I then hid the sort order field so it wasn’t shown, and that left my desired columns in the correct order.
Hope this might be of some help