Hi, what type of system do you have is it gravity and mains fed?
Ok, it sounds like the big valve is a drain \ pressure release if that is resulting in water going out of the overflow.
Best thing to do is go back to basics, and work from there.
Can you confirm, cold water is running through all taps (kitchen and bathroom), and that there is water in the header tank in the loft?
From what you have explained this looks to be gravity fed system, from looking at the pipes going to the hot water tank does this look like the image here?In order to stop hot water there are two ways, the direct way is to switch off the valve which comes off the to of the water water tank (labelled low pressure hot water on the diagram), however the other way would be to turn off the inlet from the feeder\header tank in the loft, this just takes longer as you have to wait for the pressure to reduce and lose a little more water.Can you locate the two isolation valves one being the inlet from the loft, and the other being the outlet to the hot water taps?