Usually on the side of the metal disk there is a small hole with an adjuster screw that holds the metal disk onto a spindle which pushes the plug up or down. If you can see this try and tighten the screw inside. If not, then you would probably be looking at removing the bath panel or front fixtures to gain access to the rear.
Hi fever what sort of long wall have you got because if you can't remove the bath then you may have to go from the other side this includes of course an external wall easier than you think if it is cavity, drill all round bricks to remove then longer drill/chisel inner course then re build after repairs internal studding a bit easier than removing bath disturbing tiles etc HTH Tez