It's becoming marginally clearer

The big tank has an exit under the plank on the far side. There are going to be various entries from various surface drains all of different heights. These will all drop the surface water into the bottom of the tank. Quite why the sewerage (it doesn't look right) pipe goes across rather than round still baffles. Perhaps more will become apparent over time. The blockwork is hollow and, together with rebars, filled with concrete, vibrated down. This forms a waterproof tank, so I understand.