lcwalker, have a search on the forum for 'rain water harvesting', and look on Matt's DIY site (google for DIY WFP), I can't give a link because it's against forum rules, but in the WFP DIY forum there's a lot of stuff about rain water harvesting.
Or you could start up a new post about it?
You could also contact Elite Mike from the DIY WFP forum by PM, or Jeff Brimble; both have done this and know the score, and both are helpful friendly guys (Mike lives close to me, so I know him personally).