I’ve not had to deal with this problem to date Angus. However, had I been faced with such a situation I would use WD40, making sure that the whole circumference is ‘soaked’ and allow to seap down overnight. The next day, place the key in place and with a hammer tap the key lever in the clockwise, ie tightening direction, three or four times max. Then reverse the action by tapping to unscrew. Ideally, this should precede application of a heat gun. Worth giving this one last go. Good luck.
The membership association for owners of Dufour Yachts