I'm still tempted by the idea, I've got a spare cistern valve on a drinking
trough somewhere, I can't think where it is at the mo, I'll maybe have a
look for it later. Thanks for the (one or two) helpful replies!

Julian.

Your "2 cistern" proposal would break the law by deliberately
contriving to cause
the meter to read incorrectly:

-----------------------------------------------------------------------------------

The Water Supply (Water Fittings) Regulations 1999

PART II

REQUIREMENTS

Restriction on installation etc. of water fittings

(2) No water fitting shall be installed, connected, arranged or used
in such a manner that it causes or is likely to cause-

(i) waste, misuse, undue consumption or contamination of water
supplied by a water undertaker; or

(ii) the erroneous measurement of water supplied by a water
undertaker.