A multitude of leaks! The overflow is running because the water level is too high, probably due to a leaking float valve that is not shutting off. There should be no water coming from the copper cylinder - the leak could be coming from the pipes or immersion heater or maybe from the base of the tank itself if its elderly. Think you should call the installer back with the boiler leak though - it could be coming from almost anywhere.
John
January 2008