Depends on the cylinder - some setups have a thermostat 'strapped' to the outside of the cylinder. You are going to need to follow the wiring and see what you have.
Now, the stopcock - what type? Is it a wheelvalve? If it is and is on strong pipes (ie, not plastic or 15mm copper), get some mole grips on the shaft and try turning it.
Can you shut off water to the stopcock?