i have had this before, i rounded 2 bolts off

i used 1/2" socket and bashed it onto the bolt head, it soon came off. but as said, the socket wont come back off, so just go buy some cheap ones from halfords that you can just throw away.

Also another tip, bit late now tho. but i find a 13mm socket on the gearbox bolts and starter bolts to be just a fraction to big, so i just use a 1/2" socket now, its a perfect fit, and i have never rounded of starter or gearbox bolts since