I'll second that. Rainwater diverters are crap.

Is that ALL rainwater diverters? Every single on in the whole world?

Better have a water butt that overflows into old drain

What are you going to fill it with if not the downpipe?

MM

The down pipe ,but all the flow gets fed into the water butt even when it
is full.
At that point you need to arrange for it to overflow to go down the drain
that pipe originally fed into or somewhere else like a pond or flower bed.

Basically the Butt is in the flow path all the time whereas the diverter if
it is installed correctly only feeds the Butt till it is full and then the
flow resumes going down to the drain.

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.
GH