A preform for forming a composite structure, such as, but not limited to a panel for a building, house or the like includes a body substantially formed of a reinforcing material. The reinforcing material defines one or more compartments or sections. The compartments are fillable with a granular filler material through which settable liquid matrix material is impregnatable to form the composite structure. There is also disclosed a composite structure, a panel and related methods of forming the preform, the composite structure and the panel.