water coming out of overflow pipe

[cr0ft]

Depending on what's gone wrong inside the tap you could have a situation where the cartridge is passing water out of the spout and also passing some mains pressure cold water back up the tank fed hot water supply and into the cylinder then back up into the loft tank. Basically the mains pressure pushes the tank fed hot water back up into the loft tank, filling it up and making it overflow.