On Mon, 4 Jun 2018 19:46:13 +0100, Nightjar wrote:

As everyone expects that just about all taps are connected to a
potable
mains water supply, I would suggest a clear notice on the external
tap:
"Rainwater - not for drinking".

I would not expect every tap to supply potable water. The law only
requires that there is one tap connected to the mains. In a domestic
premises that should be the cold tap over the kitchen sink. All others
should be treated as likely to be fed from a cistern, unless you
definitely know to the contrary.

Until a few years ago *none* of the taps here where on the mains, the
mains only went to 3 x 50 gallon storeage tanks in one loft. A few
years ago some of the hot taps went onto the mains. A couple of
months ago some of the cold joined them.

--
Cheers
Dave.