1. A method of controlling interruption of execution of tasks running in a multi-processing computer system to permit user intervention, said computer system including operating system means capable of running a plurality of tasks simultaneously, a repository of data identifying a preselected plurality of tasks for interruption and identifying the user defined circumstances for such interruption, and a plurality of task caches for storing subsets of the repository data for respective tasks, said method comprising the steps of: during an initialisation phase of any task, scanning the repository for data relevant to that task; storing such relevant data in the respective task cache; executing the task; determining when a defined circumstance for the task arises by reference to the respective task cache; responsive to a determination of a defined circumstance for the task arising, interrupting the task to enable user intervention, including updating of the repository to redefine circumstances for interruption; resuming execution of the task; and following resumption of execution, refreshing the respective task cache with any relevant repository updates.