Upon taking my brake calipers apart, I have come to a problem with the rear!

The allen stud which holds the pads in place is rounded off and I cannot get it out..

Ive tried grabbing the visible stud with pliers and mole grips but nothing really has enough grip on it.

Does anyone have any tips to help, and does anyone know if I can buy a replacement one afterwards?

Thanks in advance

you can get a replacement one after ok.

have you tried applying heat to it or using a penetrating oil? those methods are good to start with