A very enjoyable solve and I have the event and venue, but there are still 3 clues I cannot parse...
18a - Punched useless forward (4)
My answer means "forward" and I've tried adding letters before and after but I'm not seeing how 'punched' and 'useless' fit in.
9d - Tree we’re climbing shortly on edge of wood in (En)field (8
Definition is "tree" and I can see most of/'shortly' of "we're" reversed/'climbing' but don't see how it fits in with the rest.
7d - King Canute previously slaughtered legendary beast (7)
This looks like a straightforward anagram/'slaughtered' of a usual abbr. for "king" + "Canute", giving a "legendary beast" - but if so, then what part does "previously" play?