This video shows you how to override a zone. Some properties don't make use of all the zones. Certainly ones that were built earlier. It is possible to change it however beyond the scope of this guide.

To work out which zones are which simply override a zone and go and see if the radiator or tank is on. You will hear the tank click when it comes on. You may need a friend to help with that bit.

If you find you can't get one of your heaters to control it could be because the switch is off. By each heater you will find a wall switch. One shuts the heater off completely and one allows it to be controlled by the controller. If this still does not work its likely the zone is not used.