This is from the ultimate 60ff, if your boiler is the same model it should have the pump wired as shown, if the pump is running on its own when you call for ch then you have miswired it.
I doubt if the actuator head is faulty if its new.
Did you check the valve spindle for stiffness before fitting the replacement head.
I know what I would do! I would remove the grey wire from the terminal block and supply it with 230v from a separate source.
Then switch on CH only, there is no problem up to mid position so its a question of 'does it work or not'
If it does't not, then suspect 'head' or valve spindle.
If it does work, then problem is the wiring.
Someone had a similar problem and found a poor connection at the HW OFF terminal in the programmer
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