1. A method of diagnosing a fault within a network comprising a plurality of resources each of which has at least two operability states, the fault being manifested by failure of a service at a first network node and requested from a second network node, the method comprising steps of: identifying resources providing at least one candidate network pathway between the first and second nodes; for at least one candidate network pathway, listing failure modes of resources identified in respect of that pathway; selecting from the listed failure modes, a first group of failure modes having a probability in excess of a first probability; probing the network to establish if any of the first group of failure modes is the cause of the fault.