Well, that's exactly what happens with the diverter I bought. But it
has to be installed at the correct level compared to the inlet on the
butt.

Pros and cons for both methods.
Full bore can be useful if a large number of butts are linked by piping of
a suitable size and all have got low after a dry spell, one good thunder
shower can fill them in minutes whereas with a diverter and the small pipe
connecting it to the butt a lot of water will overwhelm the diverter and go
down the drain to waste.

As long as I get a full water butt after a rainy spell, that's all I
want.


Fine. Im not trying to convince you to change,just answered your question
of where the water in the other. method came from.
We use both methods Diverter on the sheds and greenhouse, full bore on the
house .

With a lot of blueberry plants and a pond to top up both of which would not
like the mains water we have 5 butts connected to the full bore . after a
dry spell it can take a while of gentle rain to fill them and raise the
pond level, so if you get a heavy but short duration summer downpour I want
the lot,not see some going past the diverter and into the soak away because
that link pipe isnt big enough.
If their is too much the pond and bog garden absorb it.

GH