Not nearly enough capitals for a full rant….
…..but seriously, we had suspiciously high bills when we moved into a flat. Turn everything off, and see if the meter is still going round (ours was). Then you speak to the water company. We couldn’t engage ours, but they managed to engage the landlord, and it turned out there was a very large leak under the driveway (it also explained the deepening crater in it!). Landlord’s responsibilty, and he eventually reluctantly fixed it. We did eventually get some of the hundreds of pounds we’d paid back.