Various approaches have been used to identify possible routes for improvement of patient flow within an emergency unit (EU). One such approach is to use simulation to create a 'real world' model of an EU and carry out various tests to gauge ways of improvement. This paper proposes a novel approach in which simulation is used to create a 'perfect world model'. The EU at a major UK hospital is modelled not as it is, but as it could be. The 'efficiency gap' between the 'perfect world' and the 'real world' demonstrates how operational research can be used effectively to identify the location of bottlenecks in the current 'whole hospital' patient pathway and can be used in the planning and managing of hospital resources to ensure the most effective use of those resources.