In message l.net,
Dave Liquorice writes

The valve needs the spring in the actuator to close. If you manually
close the valve does the flow stop? Perhaps the ball has
disintegrated. Does the actuator motor open and spring return when
not on the valve?

Having discovered the fault on Sunday, I put it all back together,
awaiting a new head, and now it seems to be working perfectly. I can
only think that I dislodged something, or eased a sticking part. Still
awaiting the replacement head but, with three motorised valves here, it
will come in handy one day.
--
Graeme