Leaking Water Tank

I recently found that the water tank in my 5-year-old flat was leaking. On speaking to a plumber and showing photos he advised that the tank needed to urgently be isolated and that it had been installed incorrectly. The developer's installers/plumbers are claiming that the photos show that the tank was correctly installed and that the tank was probably leaking due to a fault with the expansion vessel. I would really appreciate the advice of any other plumbers.