I think I've completed, and I came to this thread to make sure I'd been on the right track. I found it a bit confusing.
With apologies to those who might think I'm giving too much away, can you folks confirm the following
1) Using a 26 21 confirms the eventual entries at 1,4,10, 30,31,33, with the clue numbers providing the differences between the solved answers and the entries.
2) The 6Dn & 20 DN entries work out as a thematic pair when you operate as 1) above both ways (+ & -). 6Dn is NOT LAMOUR!
3) The normal entry must be 5 letters and spell out a recognisable partner word when the encoding for 1Dn is used.(+ or - 1)
4) My entry at 15 works out, giving some military hardware / aquaria as a possible partner word.
5) My question is...am I entering the military hardware / aquaria at 1Dn, or leaving name of the 1951 documentary short directed by Zinneman and starring Henry Fonda in the grid, and writing the military hardware / aquaria under the grid? If it's the former, there are no problems at 11& 16 Across, but 1 Across is a non-word.