The pump next to the emersion heater seems to be running constantly (even when heating and water is off at controls) and there is water running through the cold water pipes and running in to the overflow tank in the attic - can this be stopped?
From what you said it seems as if the central heating system pumping over and out of the vent pipe into the feed and expansion tank. this can be caused by a restriction or blockage in the central heading system. with nowhere for the water to circulate the water is forced up the vent pipe and into the tank. to fix find blockage/restriction and remove
Check pump situation. Some systems suck it round rather than pumping it around. If pump is on lowest part of heat source eg where it the cool water goes in, it probablty has been designed to suck rather than pump. If you pump on that line it will go back to the expansion tank. The pump should have an arrow on it showing direction of flow. On lowest part of heat unit arrow should point into unit, on highesgt part of heat unit arrow should point away from system as a rough rule of thumb. I know I did the same on my system before I fathomed it out. Hope this helps