Dave Liquorice wrote:

/snip/

Connect the bottom of B to the bottom of A,

And have to drain both when the connection pipe gets blocked. It will
at some point and taps are likely to cause a block as well due the
constriction.

I filter the water as it goes into the butts. I acquired a number of
nylon stump-socks (for someone with a prosthesis) and lashed them to the
bottoms of the downpipes. You get around a foot of filter, which has to
be removed and cleaned-out every other year or so.

You can fit filters to the connecting pipes too.

Later, when my system is up and running, I'll put the pics and diagrams
up on the wibble.

--
Rusty