If you have a green cylinder, then it will be fed from a cold water tank. Sometimes this will be on top of the hot water cylinder itself, you will see there is a sort of waist where it is attached, about a quarter down from the top, and if you feel on top there is a removable lid, and you will hear water running after you run the hot tap. If not, it might be on a high shelf in the airing cupboard, or, in an older building, there might be a shared tank in or on the roof.
Does your flat have gas? Does it have radiators or storage heaters? How old is it?
The cylinder may have an electric immersion heater or two. If so, there will be a round plastic or metal cap about 80mm diameter in the top or side of the cylinder, with a cable coming out of it probably going to a switch on the wall. Switch this off and see if the hot water gets less scalding later in the day, and then runs out.
Photos of the cylinder, showing the pipes and cables round it, and of the thing under the sink, will help.
Look at your electricity meter. How much have you been using, averaged per day?