Central Heating Overflow Tank

Got home about 8 this evening to be met with sound of downpour onto the conservatory (leaking) roof. But it wasn't raining. For reasons I dont understand the hot water cylinder is overheating and overflowing into this tank. The blue ball valve had slit and come adrift from the spindle and there was  water everywhere. I spent over an hour standing in the loft with the plumber from Dyno Rod holding a torch so that he could see what he was doing to sort it out. Not in bed till 1130. Not enough sleep.