There's a screw broke off in the tripod screw bit
Not the most helpful of things to do is it
If there is a bit sticking out carefully cut a slot in it with a hacksaw and unscrew it.
O sht, if you cant twist it out youll have to tap it out.
Nothing sticking out! Damn
I have a dremmel at home but I'm in Cornwall
Even with nothing sticking out you could with enough care, chisel a slot just the right size for a screwdriver. Obviously if it's much below the level of the tripod mount, it becomes harder to do and then it probably would have to tapped out.