Voting is a critical component of any democratic process; and electronic voting systems should be developed following best practices for critical system development. E-voting has illustrated the importance of formal software engineering in the development of complex systems: poorly engineered and poorly documented voting systems have had serious negative consequences for all system stakeholders. It is clear that the formal verification of e-voting system models would help to address problems associated with certification against standards, and would improve the trustworthiness of the final systems. However, it is not yet clear how best to carry out such formal modelling and verification in order to leverage the compositional nature of the problem, and manage the complexity of the task. The choice of modelling language - for expressing the high level design and architecture of an e-voting system - poses many problems due to the complex mix of requirements that such a system is required to meet. Different modelling languages are more-or-less suited to the verification of different critical requirements. Thus, we report on a mixed model approach: where we address 3 different types of critical requirements using 3 different modelling languages and development strategies. Firstly, we report on network quality-of-service issues that are analyzed through simulation models. Secondly, we report on functional correctness of a counting process that can be validated through algebraic techniques. Finally, we report on the use of formal refinement to reason about the correctness of design steps when adding detail to an architecture model. To conclude, we acknowledge the main problem that arises from such a mixed-model approach to architecture verification: how can we be sure that the different models are coherent when we integrate them in a final implementation?