Potterton Performa 28 Heating?

My Potterton Performa 28 is working and carrying out functions of heating water and central heating. However, it has now started switching on central heating even when it at hot water only setting. It also randomly switches on heating unless it is set to "0". I have replaced the timer switch, thermostat, and PCB and it still not fixed problem. Any suggestions would be greatly appreciated. Thank you.