Having trouble turning off water to shower, any tips?

It's a gravity fed electric shower and situated at one end of the house, its a bungalow, the boiler, main bathroom, tanks are up the other side. When I'm in the attic, there seems to be only one pipe going to that side of the house and it goes into the ceiling exactly where the shower is. It has to be the right pipe, right? The other end of the pipe goes right into the tank and there is a tap just where it comes out of it. I open the filter on the shower and the water gushes out, I close off the tap at the tank, I presume clockwise is off, but the water is still gushing out and doesn't seem to be getting weaker. I've waited 5 minutes and no change, still pouring out. I can hear the tank refilling too as soon as I do this so it's definitely coming from the tank. Is the tap broken? It feels ok but how would i know. Anything else I can do to stop the water going into the shower? If I turn it off at the mains and let the tank run dry is that dangerous or could it cause problems? The house is about 20 years old.