Ideally it's head off and a Wurth Time Sert, but they're not a cheap kit, plus new head gasket etc. Faced with a similar problem a few years ago, I tried a Vcoil repair kit (Helicoil copy) without removing the head and all is well a few hundred miles later. Obviously a risk with cutting a thread without removing the head and potential to wind the insert out when removing the plug, but my experience has been good so far.
Andy