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Quite a challenge, as to expected from Hamish. Not quite sure that I see the logic mentioned in the final sentence of the preamble, but everything fits.

Now got it. The clue lies in the word "added"

I created an interactive grid should anyone find it useful. Here is a link...

https://crossword.info/jono/Genius_250_Soup

Finished except for 18d . Went back on tree being hacked down(8) I have r?v?r?e? . Could be reverted or reversed but can’t parse either. Obviously v is the extra letter.Thanks in advance.

Jadzia, I took 18d. as 'reverted'. RE, on,+ TREE anagram, (being hacked) + D, down.

On is re, then tree* + d

I think!

Thanks folks.

very clever, Hamish - nearly 21d by the mathematics, but it was worth it for the wit of it ...

my online entry form isn't there and it says the deadline for entering is past! I've tried rebooting with the same result ...

21ed, that should be.