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You have removed the tang on the locknut washer. It's bent over so the nut can't come loose
The only thing I can suggest is to use an induction heater on the nut itself. The localised heat applied just to the nut by the heating coil may be enough to break the seal on the thread between the nut and the shaft. A mate used one on the nut of a Muirhill tractor after trying everything else and the but just came undone!Problem will be finding someone who has one as they are mega expensive to buy. Maybe try a local garage/mechanic. I'll ask my mate where he got his from, he lives in Buckinghamshire though so a long way from you.I really wouldn't want to try and drill it off considering where it is and the shaft it is attached to.