Malcolm Stewart wrote:
I'm getting a bit fed up with the authorities telling us that rain in summer
is no real help for our struggling(?) water supply industry. (Due to
evaporation, transpiration from trees etc.)
snip
when one is full, others are then filled. In Milton Keynes we have a number
of balancing lakes designed not to store water for local usage (other than
as nature reserves, boating lakes), but primarily to reduce flooding down
stream of Milton Keynes.

Consider your average sized bit of decking/patio.

If this was constructed so that you excavate 0.5m down, and put a membrane
down, with pillars to hold up the decking/patio above, another membrane,
then the patio/decking/...
So you've essentially got a huge sealed plastic bag under the
patio/decking.
A 3m*3m*0.5m tank = 4500l.

Ideally you'd probably like to size this so that it fills completely
with whatever spare water you've got in the wetter months, for use in
the drier.