Fauly Potterton EP200 Programmer?

Even though my programmer is at OFF and the room thermostat is LOW, my heating circuit is on at the same time as the hot water. This doesn't matter in winter, but does in summer. Sometimes switching the whole system off at the main electrical power switch clears the problem. Is the problem in the programmer?
The progammer is a Potterton EP2000 and is about 30 years old. Is the new looking replacement a straightforward replacement?