Hi Conor.
What hull No. is your boat?
I know on early BRs the yard plugs were routed, not turned. And, they were only glued in with marine sealant. A shoddy job.
Meaning, these plugs were more like disks, so the length of the plug protruding into the carbon spar was too short, about 10 mm, and the sealant soon gave up and the plug fell out. Happened to me. On my first outing.
Apparently, your own end plug holding the jaws did not fall out but just lost grip, and rotated inside the hollow yard. IMO you can glue it back in, and secure it into position with two or three s/s screws. That'll work, until it loses grip again, usually happens at the wrong moment, i.e. in a wind.
My advice: check the material and length of the plug that you have. The material should be mahogany or better (teak is best), and the length protruding into the hollow yard should be 1.5 - 2 times the open inner diameter. Meaning, with the inner diameter of the yard being close to 50mm the insert length should be at least 75mm. And there should be a good rim on the plug, min. 1 mm wider than the outer diameter of the carbon tube.
If you do not have a lathe yet, go get one. A cheap one will suffice. Very useful if you have a BR, as it allows you to turn yourself proper length plugs for cylindrical carbon main mast, foot and top, mizzen mast, foot and top, gunter yard, foot and top, and carbon self-tacking jib boom, front and clewside.
C.