There has been significant recent interest in local electricity trading platforms, and particularly in the application of blockchain technology for distributed, or peer-to-peer energy trading in local energy communities. Several projects worldwide have demonstrated this concept on a small scale in Low Voltage (LV) distribution networks and microgrids. However, previous work in this area has not sufficiently addressed the potential impacts of peer-to-peer energy trading and other local electricity trading mechanisms on the control, operation and planning of the electricity distribution networks. Accordingly, this paper presents a methodology for the co-simulation of power distribution networks and local peer-to-peer energy trading platforms. The co-simulation approach is based on detailed network analysis of the distribution system using the open-source distribution system simulator OpenDSS interfaced with a blockchain-based energy trading simulator. The presented co-simulation approach is demonstrated using a case study of typical European suburban distribution network.