This paper presents an approach to the modelling and simulation of highly distributed power systems, in the context of existing UK distribution networks. The model is part of a framework to assess the network impact of high-penetrations of micro-generation, as well as demand side management (DSM) measures. This framework supports the representation of the 11 kV and low voltage distribution network within a geospatial model, together with an integrated load-flow computational capability. A domestic electricity demand model is incorporated within the framework that generates synthetic demand profiles within the network. The framework is used to investigate the impact of distributed generation for a case study of a regional UK town, together with the surrounding villages and rural locale. The scope includes primary distribution substations, 11 kV feeders and a low-voltage distribution network, serving approximately 35 000 domestic properties and commercial premises. This case study also includes the measurement of minute-by-minute residential electricity usage within a number of individual domestic houses and distribution sub-stations.