The recent discovery of superconductivity in a series of iron pnictide materials has led to the anticipation that they might lead to new insights into the origins of high-temperature superconductivity. The first total scattering + RMC studies of this family of materials is proposed, focussing on examples from the "122" family with the ThCr2Si2 structure: the "parent" phase BaFe2As2, the isoelectronic analogue with disorder on the alkaline earth site: Ba0.6Sr0.4Fe2As2 (like BaFe2As2 a non-superconducting antiferromagnet at low temperatures), and the superconductor Ba0.6K0.4Fe2As2 (chosen to have the optimal Tc of 38 K). The three systems will enable a systematic comparison of the effect of disorder on the local structure, magnetism and electronic properties of this family.