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Now smack it with a small hammer or the handle end of an adjustable spanner, etc.
Fun fact: what you call "adjustable spanner" we germans actually call "Engländer".
Thanks for your help!
No luck so far, but I think this will work once I find "something strong" that will fit the hole perfectly.
Actually I read (and think) something like sort of a twisting motion would loosen this very quickly,
alas I can't find something like this here
Slot something strong like an allen key into one of the two larger holes; it'll go in maybe 5mm but not as far as the threads as you choose one which is just small enough to fit. Now smack it with a small hammer or the handle end of an adjustable spanner, etc.