Flakey, the first place to start is the pressure loss from the heating system, check the boiler and the discharge pipe that goes outside, look for any water, if you want, tape a water bottle over the discharge pipe outside to confirm it's not letting by, once you have confirmed it's not the boiler then that leaves the pipework and rads, it will be the amount of time the system pressure takes to drop that will indicate how bad the leak is, if you can't see any damp patches or drips around any rad valves then you will have to get someone in to help identify where the leak is ie.. thermal camera or pressure testing individual circuits, a good plumber can help you here, unfortunately you must deal with the pressure loss first, once that is dealt with then you can decide what kind of flushing is needed by asking the plumber what the water is like and if he feels it needs a big flush or little flush before any inhibitor is added, it's not till you get to this point that you are giving the boiler a Chance to work properly, there may not be to much wrong with the boiler if it's firing to generate heat for the hot water cylinder, it may just be a external control fault, like the boxes(zone valve) in your hot press, hope this helps, Gary.