H and E tank overflowing?

The central heating H and E tank continues to fill up to the level of the overflow outlet pipe even after the seal, the ball and eventually the balcock assembly has be replaced . I am reasonably confident that the ballcock mechanism is operating ok as when some of the water is removed from the tank to a level where the valve is still closed by the action of the floating ball and the level is one inch below the overflow outlet, the water level eventually rises to cause the overflow again. The water in the tank can at times be warm. This would suggest that the incoming water is coming from the central heating system. The hot water from the taps in the bathroom is very hot. I have replaced the immersion timer switch. The installation is 40 years old.