When you do manage to get it off, do the "mod" of twisting a piece of wire round each of the retaining hooks that hold the seat in place, and looping it out of the holes under the rear seat. Then just tug on the wire to release the catches - no need to use the (slightly bent in my case) key.
Had same with mine remove black undertray lining rear wheel arch this is done by removig screws that hold number plate holder onto the undertary and I think two other screws about two thirds of way towards suspension side of undertray. You will need to support the tray once removed. You should then be able to remove what ever is obstructing you turning the key good luck
Removing the undertray worked, lock now released, would appear a flouro vest stored for the last 6 months at the front of the seat jammed the lock at the rear,