Having problems with my hot water - it won't switch off and is constantly asking the boiler to fire up. Water is VVVV hot. Have changed the programmer on the boiler - to no avail so think actuator valve is not working - seems to be sticking as i can move it manually. I have drayton valves in the airing cupboard. Can I easily change or stripdown the valve for the hot water. Are they expensive to buy or difficult to fit a new one??