There's a reason that proper rattle gun sockets are 6 sided.could almost swear that bolt has loctite i used the rattle gun and it would not budge i kept at it thats when it stripped, i tried a 12 point socket
If it's rounded, then a chisel is the only thing to use, but get a new plug for refitting.6 point socket it just wont come loose
It sounds like you've bought a Freelander that used to be maintained by a heavy plant mechanic.