Thing about plastics Wren is not 'will it fail?' but 'when will it fail?' as fail it will. As RB says sunlight plays a part and to resist that effect plastics are pigmented black or dark colours. The other thing is ozone in the air, high near the coast. Also, as said, what was in it, which can cause 'environmental stress cracking' which basically is any area under load will crack, like the tap. In this case it is probably ESC from the contents or something spilled on or used to clean it.
Can you get a pressure washer into it from the filling point?