Sigh. Spend an hour looking for a solution, fall back to a post asking for help, find the solution 5min later....
For anyone else searching in the future:
From paperless-ngxs settings page (logged in as an admin)
'Open Django-Admin' > 'Paperless Mail' > 'Processed Mails'
Select what you'd like, then delete.
Glad I could help :)
At least this post wasn't a total waste...