Selgas
I’m with you on this one Dunbar – the cylinder is leaking – turn off the gas to the unit and the cold water feeding the cylinder, drain down the cylinder and wait for the Plumber to arrive to replace the water heater.
No valves should be fitted on the outlet of the cylinder ESPECIALLY if it is gas fired as the heat recovery with gas is way faster than electric and if both valves were fitted and shut down with the gas still running a time bomb is sure to be the result. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
