Potterton Promax 24 HE - Heating without hot water problem?

Hi, I have a Potterton Promax 24 HE boiler on a pumped system. Since the start of the recent cold weather I have been unable to get the heating on without the hot water being on. Once the water is hot enough the boiler shuts off regardless of whether the heating has reached the desired temperature or not. I have changed the programmer and heating thermostat without any joy. My next step is the control card but don't want to fork out £140+ if this isn't the problem.