I took the actuator apart and burnished the contacts. The mechnisim was a bit rusty, looks like the valve leaks a bit. It has been "working" until today. It is in constant motion again.
"Working" means with the central heating turned off it would cycle through to the central heating...
The actuator valve which determines whether the combi boiler is heating for hot water or the central heating system is in constant motion. With the heating system turned off at the boiler and the hot water not demanded the switch still runs. I realized this when it started making noise while...