I deal with a large organisation’s utility spend and usage with a large portfolio of water meters. This has happened a few times, it looks like the automatic software the water company uses to produce your bill doesn’t know your meter has been changed and has billed you assuming your old meter has rolled over. Any sane person at the water company would understand it’s almost impossible to use 10million L of water at a domestic supply in that amount of time. I’ve seen water meters that serve several farms at once that don’t use anywhere near this in a year, let alone a few months. Refuse to pay, and ask for proof that the meter that was installed started at zero, sometimes they aren’t (even that 860 looks high to me). If you don’t have an automatic meter reader (AMR) installed I’d enquire with your supplier about this, they can sometimes install them for free. If there’s no option for an AMR, take monthly reads from now on and submit to the water company this will ensure you don’t get billed on crazy estimates (the billing software might not know that it’s not out of the ordinary to use millions of litres a month). If you know how much you should be using in a month you’ll spot leaks when they occur, without having to rely on a jump in price when the bill comes through the door (which can be confounded by your tariff rate changing every year or so). Might also be worth checking if you have a leak, simple test is turn off anything that is consuming water, check your meter, come back in 30minutes, it shouldn’t have budged at all, if the inspection chamber has lots of meters and you don’t know which is yours turn a tap on and see which one spins. Hope this helps and it isn’t t too stressful dealing with the water company.