Leaky bathroom tap

The house is now hot water free thanks to a leaky bath tap. Its pouring out like Niagara Falls and i can hear the water tank in the loft continually refilling!
Will there be some sort of shut off valve directly under the tap so i can then simply replace a washer or will i have to shut off the entire water supply to the whole house?
Our heating system is , i believe, a regular gravity system. Its not a combi on demand system. That much i do know.
Thanks.