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Defoxster-
Put a scrap (ikea flatpack) allen key in the bolt then heat the free end of the allen key with a blow torch. It keeps the heat where you want it.
Let it cool a bit and spray penetrating oil on.
Wait 30 mins and repeat.
When you feel the moment is right try to tighten the bolt a fraction, if you round it off you still have the undo faces undamaged. If it tightens a fraction then undo.
Dibble
Dibs I’ll sneak past my wife