It could be that your toilet is leaking or wasting water, perhaps due to an overflowing cistern. And if you're billed by meter, rather than upon rateable value, then you will be hit.
If you have a toilet that is ten years old or less, then it could have what is called an "internal overflow flush valve" fitted, which is great as it removes the unsightly overflow pipe and saves drilling a hole in your house for it.
But it does mean you need to look out for the signs that your cistern is overfilling. There are two signs this is happening and they are discreet.
One of these could be water constantly trickling on the back of the inside of the pan. This may be very slight or a continuous flow, you may even have a build-up of limescale if it's been going on for a while.
You can check this by inserting a flush freshener block that is put in your cistern, which should only dispense coloured water when flushing. Wait 20-30 minutes after putting this in, and do not flush the toilet during this time. Look in the bowl - if the water has coloured, then there is a leak.
Another sign of an overflowing cistern is that you may hear it. If you have high water pressure and your fill valve is letting in a lot of extra water you may hear the flow of the pipe faintly. Don't assume it's not just because you can't hear the water landing however, as many fill valves have a silent fill system. This means that the water enters from below the water level rather than dropping to the water surface.
Other visible signs of water overflowing or leaking are bubbles and/or indentation in the water in the bottom of the water closet pan, as the overflowing water meets it.
As a result of water overflowing into the pan, more water is being "consumed" and will increase your bill, which you are still responsible for paying. Double check for all these signs, and if you think that there may be a problem with your toilet, please report it to our repairs team.