Had a look at the "First Flush" system you mentioned. I had a house in Australia which had a basic system like that attached to each gutter exit however it did not have the release valve to automatically release the collected water. If I was doing a new installation I might consider such a system. "Mr Bob" saw a system that someone had done in Buriram which utiliser a tennis ball, It may be a home-made version of the FF system. I will find out some time. I have been annoying the hell out of him lately, I am reluctant to contact him again anytime soon!!