Sorry gents missed this being at the yorkshire meet
assuming you DONT have a combination boiler and you Do still have a hot water cyl.
You will have a pump on the main flow from the C/H boiler to the hot water cyl. Just after the pump you will have either a single "three port" valve that directs the water to your hot water tank or your heating system. if not you will have two single valves that will open when hot water or heating is requested by the system.
If the water in the tanks in the loft falls too low due to a stuck ballcock, the first thing that will fail is the hot water.
If the pump sticks you may still get hot water due to the water gravitating.
If the three way valve fails to open you can tell. there is a slide lever on the side of the valve. when it is open for heating the lever can be moved from it's rest position across to the otherside with minimal effort. if it takes effort to move this and you can hear it pulling the motor with the lever then it is not opening.
To check the pump. You will see two valves either side of the pump with square shafts, turn these clockwise as if turning a tap off, The grunfos pump has a chrome centre cap, remove the centre cap. A small screwdriver will fit into the end of the shaft to ensure the pump is free. It is a magnetic induction pump, no gears so they are prone to sticking after a period on none use. Once you have spun the impeller with the screwdriver a few times, replace the cap and turn the valves back on then try your heating. a trusty screwdriver between the pump and the ear will show even the untrained if the pump is running.
For the 3 way valve: With the system set to H/W only, pull the manual lever in the side, you should feel resistance on the lever but you will be able pull it across, when you have done this release it and it should return. Next turn off the H/W and the C/H on. wait a few mins for the valve to position itself. then pull on the same lever and you should feel no resistance at all.
If you need a new pump or three way valve let me know, I have access to a couple at trade prices.