Having arranged the water storage for garden use, I noticed that there were other down pipes that could possibly reduce the amount of water caught. If I could reduce run off without increased chances of flooding, we should be able to collect more water per down pour.
The idea is that if water could be dammed at two points, we could increase the volume of water able to be drained into the barrels.
The catchment area is approximately 150 square meters.
The spouting is 5cm deep before it overflows into the house, and the dam wall is 2cm, which means that in a significant rain event the water can spill over.