Inventory control models which provide co-ordinated control (or joint replenishment) of families of items are very useful in practice. An important component of these models is the algorithm for allocating a total replenishment order among the items in the family. In this paper the allocation algorithm from a new class of co-ordinated control models is discussed. Inventory position is modelled as a diffusion process, and both continuous review and periodic review situations are considered. This new class of co-ordinated control models has been shown to outperform existing models (IBM's IMPACT Inventory Control Package).