Help i'm struggling on this.

[vorsprung durch technik]

As above, i'm trying to undo the cars original hard brake lines on the S6 saloon to change the flexi hoses and they're rock solid. Ive so far tried freezing them and can slightly turn the 11mm nut on the brake line but it's twisting the brake line also. This has happened on both sides at the rear. I have spare perfect lines removed from the 2.8 quattro donor car but have just realised if i'm going to replace all the rear lines to just under where the rear seat is i'll have to remove the fuel tank. Thats a big no no as the car is on the road and i can't move it. Can anyone suggest a way around this problem?

Pic of what i'm talking about.