A computer network comprising a plurality of terminals (1 4) connected via a communications network 5 is provided. In each of the terminals (1 4) application software (15) for generating XML source documents is provided. XML source documents created using the application software (15) are then sent to one of the terminals (4) having stored therein a consistency checker (10). The consistency checker (10) processes the source document together with a set of consistency rules (14) to generate an output presentation identifying data within the source documents which does or does not fulfil the requirements of consistency relationships defined by the consistency rules (14).