A modular fibre reinforced plastic flange (105) for a structural composite beam (100) which comprises a body (142) formed of a plurality of elongate elements (40, 140) arranged in an array, wherein the dimensions of the body are substantially determined by the number and arrangement of the elongate elements in the array and wherein at least two of the elongate elements comprise different materials, and a skin member (120, 130) at least partially surrounding the array. Also, a structural composite beam (100) comprising the modular fibre reinforced plastic flange (105) and a shear web (50) connected to the skin member (120, 130) of the modular flange. A method of making the modular flange (105) and beams (100), and a kit of parts for making the modular flange (105) are also disclosed.