RJH wrote:

I'm not sure if I'm using the extractor incorrectly, or it's just not very
effective. Thinking now about cutting a slot in the head and trying a flat
blade screwdriver. I don't think they're outrageously tight. I can access the
other side - but I don't want to damage the thread in the sheet the bolt
screws into.

Suggestions appreciated.

big rubber band wedged in what's left of the head to let an allen key,
or the extractor get a bit more bite?