I have the same problem as was originally posted, however don’t know the valves involved in the system. The setup is the same as deckers and I have just started to experience the exact same problem with the heating being on, even with thermostat off or on low, controller switched off for CH and HW.
My basic understanding is, I have a potterman boiler in the garage, a water tank in the loft and a heater tank in the cupboard on 1st floor. I moved the property a year ago with no issues and haven’t needed CH for 6months and today when I turned it on, it wouldn’t turn off. After trying all the basic of hall thermostat, timer settings, boiler reset etc, I had to turn the boiler off at the mains in order to stop CH.