Dave Liquorice wrote:

.. Also automatic (if required) change
over between mains and harvested water might be fun and you really do not
want any back siphonage of harvested water into the mains...

How bloody irresponsible an answer.

It doesnt sound like 'fun' to me. I take very seriously any threat to
Public Water Supplies. Your local Water Inspector might have something
to say about the possibility of a criminal offence, which it could be.