Midpoint actuator or valve issue

The issue I am having is that the central heating is not turning off when scheduled to. It appears to be a fault with either the midpoint actuator or the brass valve. The actuator stays in the central heating position so I have to remove the actuator box and turn the spindle using pliers. It’s too stiff to move by hand. Does this sound like a problem with the actuator box (which is 9+ years old) or the brass valve itself? Thanks in advance for your help.