Selgas
The fitting you are speaking of is a drain facility to allow the cylinder to be drained down or flushed out. Turn off the gas to the cylinder, then turn off the cold water going into the cylinder and open the drain valve to empty the cylinder, replace the faulty drain tap, refill the cylinder, open a hot water tap to ensure the water tank is full and that water flows strongly from the tap/faucet, close the tap/faucet, relight the cylinder and in about 1 hour you will have hot water again. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
