Our reader might not be helped a lot yet.
Ok you have two valves. One is connected to the hot water cylinder, one to the radiators. You shouldbe able to follow the pipes to tell which is which.
Thw way these valves work is to turn the boiler ON once they're open. They switch directly between mains live and boiler, not through the timer or thermostat. So it sounds like your valve on the HW is stuck open. Sometimes it's the motorised head/actuator which sticks, sometimes the brass bit with the water in.
If you can get the head off the valve you'll see the flattened shaft which connects the two and you may be able to decide which is faulty. If you "go there", let us know what you fnd.
ChrisR, you are a STAR. i have just read your advice. it sounded great. obviously it would have helped a novice to tell which valve is for HW the one attached to the cylinder or to the main pipes. nevertheless, once i got the clue from you to where to look for the problem, i investigated systematically (as i would in my work (human DIY, no do not go far, i mean a medical doctor, in fact consultant paediatrician!!) to find out. i would find out by checking which pipe heats when i switch it on. i did not need to do this as i first tested the manual overrides in each valve. one moved under smooth resistance and went back to spontaneously to its original position. the valve attached to the cylinder, on the otherhand, its manual over ride moves freely in any direction and into any position!!
i opened this valve, not knowing how exactly i will find out whether it is definitely defective. i would not do any thing risky. i carefully investigated it planning to buy the electric acuator head anyway for trial if i ahve no answers. then suddenly when i shook it gently for loose parts, a plastic broken piece came off. i then checked the brass bit attched to the pipe as you suggested to ensure it is not also jammed. it moved easily in both directions. i am now goining back to check the other valve (identical) for were the broken plastic piece is supposed to be. i am sorry that here were this episode ends. not to keep you susspended, but because it is to late to find a place to buy the actuator tonight. next episode will be tomorrow. i promise, and owe it to you to let you know the actual outcome. i am farly confident that you are right. i was convinced with what you said because i did not know that the valves can switch the HW directly even if the timer/programmer is on OFF position for all.