Don't jump to conclusions that the valve is faulty. I have had similar experience and was a faulty MA1 actuator. This snaps onto the valve. Unsnap it and check that the lever on the side automatically moves to far-end (cable end) for HW only, mid-position for HW+CH or far-end (furthest from cable) for CH only.
Gary
October 2007
feel the pipe on the heating side of the valve. if its hot as i would suspect the ball in the bottom of the valve is not shutting fully. you can unclip the head unit and move the ball with a pair of pliers. but the ball inside they do foul after time.
they retail about £50.00