You'll find they do fill up quite quickly and when full it automatically rediverts it back down the drain pipe. your water company might give you money off if you use one. Ours gave us this do-dad together with a water buttt, cover, and stand for a few pennies I seem to remember. The weak point on ours was the tap.Eye has bortid me water butts a rain harvesting fing which goes in me castle drain pipe to filter snd divert water to me wster butts. Yer turn a fing to harvest or not. Can't gerrum int chops so had to order online. Bin meaning to set this up fer yesrs but we only had about 25mm of rain during March and April. When it does rain again eye wants to collect as much oft me castle roof as eye can. Currently only get rain from half me garage.
Best of luck wiv it!