My brother in law asked me the other day if my back box was low (a bit personal I thought) but he meant on the car

So after having a look, it turns out the hanger brackets are so rusty that they're in danger of snapping!
I've just collected a new pair of brackets from Craddocks but I see that they have a threaded bolt attached that is tightened to the car by a bolt onto the chassis. My question is, has anyone else tackled this and managed to get the bolt undone. I'm not at home, so I can't check for access that will give me a decent amount of torque to undo, what will clearly be a very rusted on bolt.

Thanks Got - 2017 SDV8 Autobiography in Loire Blue
Had- 2008 TDV8 Vogue SE in Java black
Had - 2007 S/C in Stornoway Grey