The 2-port motorised valve on the central heating isn't turning off once
the room thermostat is satisfied and so the boiler continues to fire
until I turn the boiler off at the mains.

The actuator is a replacement Siemens RMH on a Boss Therm BZV22 body.

In the fault state, the silver lever below the valve can be freely moved
left to right and back with no resistance. If I take the actuator head
off the valve the lever springs to the left to the closed position.

When the room stat is calling for heat I assume the valve is driven open
and held open but is it driven closed once the stat is satisfied or the
hold released and springs are left to close it?

Though the fault has rectified itself a couple or three times, am I
right to assume I'm looking for a new actuator head?

--
Frank