We were talking about this very subject over on LRUK a while back and what to do about the double skin bottom. This is what I suggested: Turn the tank upside down and stuff a hot air blower in one of the holes to heat up the whole tank. Pour some Waxoyl or Dinitrol or something similar onto the tank bottom around the drain plug, where you can see the tank bottom proper. The heat should draw the Waxoyl in, under the double skin, do this until it is obvious that the whole of the double skinned area is treated. Take sensible precautions to avoid fire, do it outside and keep feeling the tank so that it doesn't get too hot etc...
I wish I'd thought of this before I fitted my two new tanks, I just schutzed them.