Technically the heapstead is the building / area around the top of the shaft, Typically it means the structure straight underneath the heastock and usually includes the headstock itself.
Its a pretty vague term and can be applied differently in various locations due to the wide variety of structures and layouts used in mining. For example at a site like Hatfield where shafts are close together you could argue that all the buildings in that area collectively form the heapsead. This might then include vent plant, compressor house and other nearby buildings.