Is there a plumber about?

I'm in trouble! My tank in my bathroom has sprung a leak. Its 4 .15am on Sat morning. My partners at work so cant help me. could a plumber tell me what to do? This is an old house-56 yrs- and the original water tank is still in it. It is a triangular tank in the corner of the bathroom. The water in this is heated by a backboiler in the fire downstairs-there is no immersion heater. I turned on the tap and tried to run the water out of the tank but its being filled by the cold water mains supply. Theres a tap or a key or a faucet, whatever you call it, under the stairs on the water pipe. It wont budge for me so I dont know do I turn it clockwise or anticlockwise? Its so stiff I'll have to hit it with the hammer and if I hit it the wrong way I'll b in worse trouble. Does anyone know which way it should turn? Are these things universal or do they differ from country to country? Im in Ireland. I have the cold tap running downstairs to stop the tank gettin too full or should It b upstar