The tank has three fixings; one on the spring tunnel, a second on a bracket welded to the inner wing (as is mentioned above) and the third is a length of round bar, one end flattened and drilled which bolts to the upstand on the valence and threaded at the top to engage with the hole in the flange on the tank. It does look as if you are missing at least one mounting. I can let you have pictures of mine if it would help but not immediately.
rgds
John