There is a new documentary out on the Tirpitz raid where one of the vets discusses the extra tanks, and there is even a schematic. http://www.imdb.com/title/tt3621966/ Not what you thought. Check it out. I just saw it on either Netflix of amazon.com. Forget which.
Thanks for that Brian though unfortunately I can't find the documentary on Netflix or Amazon. Perhaps it's a regional thing that it's not available in the UK.
Can you elaborate a little on the extra tank mystery?