1. A computer program product for controlling a source computer to update out-of-date data stored by a plurality of destination computers with updated data stored by said source computer using a computer network connecting said source computer to said plurality of destination computers, said computer program product comprising: associating code operable to associate priority data specifying a priority level with each destination computer; establishing code operable in dependence upon said priority data to establish a plurality of groups of destination computers such that destination computers within a group of destination computers share a common priority level; generating code operable to generate a plurality of push update tasks driven by said source computer, each push update task serving to transfer said updated data from said source computer to a corresponding group of destination computers via said computer network; ordering code operable to order said plurality of push update tasks in dependence upon said common priority level of each group of destination computers to which a push update task relates to form a sequence of push update tasks such that push update tasks corresponding to a higher common priority level occur within said sequence before push update tasks corresponding to a lower common priority level; and execution code operable to sequentially execute said sequence of push update tasks upon said source computer to transfer said updated data from said source computer to said plurality of destination computers via said computer network.