The problem is almost certainly to do with the motorised diverter valve that diverts the hot water in the system into either the rads, the hot water tank, or both, depending what you select on the programmer. These things tend to stick after not being used and the scenario you present is a classic - the first time (for many months) you asked it to change its position (when you asked for heating only), it probably tried to move and then gave up the ghost. Now its stuck between positions (probably). See if you can find such a device (probably in the airing cupboard near the hot water cylinder) and try listening for it's motor slowly turning whilst someone turns the programmer settings for you.