In article , Andrew Gabriel
writes

I had a ballvalve break in the toilet cistern overnight,
resulting in filling at full bore speed (and amazingly
the overflow coped, just). I don't know how long it was
running for (8 hours max), but that trebled my water
usage that 6 month period. Water company provides a
graph of usage over past 3 years on the bills, and there
was a large spike.

But it says in the literature that you are allowed 1 major loss of
water and they will adjust your bill accordingly for that. At least it
says so on the leaflet from 3 valleys.
--
Janet Tweedy
Dalmatian Telegraph
http://www.lancedal.demon.co.uk