The Environment Agency have published a postion statement on rainwater harvesting.  This covers a range of issues including the potential reduction in mains water use, flood risk management benefits, overall sustainability, greenhouse gas emissions and the applicability of systems to different areas and uses.  It contains useful references and picks out the salient points you need to consider in designing, installing and operating a system.  It does however approach the subject from a very precautionary and less than positive aspect which is quick to point out the challenges and light on the benefits.  You can find the statement and link from this to other documents at: