turn off hot water supply?

We want to install a new kitchen tap. Cold water can be turned off at stopcock no problem. We have indirect supply with cold water tank in loft, immersion heater in cupboard, with 2 red handles, one which is hot and will turn off and the other which we think is the cold supply is stuck. Do we just turn off the stopcock and empty the tank upstairs as there is no other turn off from this tank. We are concerned that we could have problems with air locks after completing the job. Any advice would be much appreciated.