Looking in her airing cupboard there was a Honeywell V4073 three-port valve. The hot water was set to be on and the room thermostat was turned down low. Feeling the flow and return to the cylinder both pipes were only slightly warm. Feeling the lever on the valve head it was loose, and moved easily from side to side with no resistance. This indicates that the valve was open for the heating.
After removing the casing from the valve I attempted to move the mechanism back to the hot water position manually. I was unable to do so and so removed the head from the valve body. As soon as I removed the valve head it sprung back to the hot water position. I felt the spindle on the valve body and that appeared to be moving with little resistance.
I replaced the motorised valve head with a new one and tested it's operation. The valve was operating correctly and heating was restored to the hot water cylinder.