I was using a double hex, found one of my dad's single hex ones from an old expensive toolkit - it seems to fit way better. It is rounded when using the double hex, but the single hex seems to fit way better and gets a lot more purchase, it doesn't feel like it's going to strip when I put pressure on it. Just needs some more penetrating oil and to sit overnight I think. The single hex socket managed to undo one of the other ones that was seized but not stripped - works way better than the double hex ones!! Thanks a lot for your help!