Loose as a goose. In fact, after the pic, I took the bolt out completely and gave some love taps with a hammer to loosen more. Got loads of caked debris out. Applying another soak of penetrative oil whilst I go get a sufficient pipe wrench.
In subreddit r/AskMechanics
In a post titled:
What tool do I use to loosen this adjustment nut to adjust toe?? It isn't flanged and only has very shallow grooves and can't get a bite with anything I have.