Posted: 16/06/2013 at 20:50
You can get kits which don't need the downpipe cutting - you just drill a hole and insert a special fitting. Here's one (scroll down to "Speedy Rain Water Divertor Collector"):
I don't know how efficient they are though. You can also buy replacement water butt tap kits - one on this page:
No affiliation with that site or recommendation - t's just the first one google found.