On Sat, 27 Jul 2013 09:03:32 +0100, Broadback wrote:

above the bottom of each tank.

There is a distinct advantage connecting them at the bottom. There is
effectively one large tank,as opposed to three smaller ones. This makes
if much easier to extract the water, either by a single tap of a
submersible pump in one tank.

But a leak anywhere empties everything but the so would siphons with
the U ends near the bottoms. B-)

Think I'd go for the cascading overflow system and taps on each tank.
Only one hole to leak below the water line. Proper taps not the cheap
plastic tat that comes with or sold as "water butt taps". The current
butt has a 15 mm lever ball valve, female BSP one side screws
directly onto the tank connector, compression the other onto a bit of
pipe with an elbow. Easy to operate and reliable.

--
Cheers
Dave.