if you get an old 12mm spanner, i say old because you might have to grind it flat abit, you can squeeze it down between the subframe and the tank, or if you can get one side a little bit loose put a spanner on the tube that runs through the tank there's too flat parts on the tube. I had the same problems, every time i remove those bolts now i put grease on the threads. :bounce3:
Hope this helps.
mark.