loft tank overflow?

In my loft there only 1 large black tank, i do have a conventional boiler system, so would the smaller tank be in the boiler system. Also within the house i cant seem to find a copper cylinder may thiis be in the boiler too, cos it is a big free standing boiler on the ground floor like the size of a washing washing. With the expansion pipe, there is a grey plastic pipe standing outside the tank but i dont know what this for? Its neither in the tank just sat next to it facing up , should this pipe be in the tank? please can anyone help?