We describe an approach to embedding a formal method within UML use case modelling. Moreover, we extend use case modelling to allow for the explicit representation of safety concerns. Our motivation comes from interaction with systems and safety engineers who routinely rely upon use case modelling during the early stages of defining and analysing system behaviours. Our chosen formal method is Event-B, which is refinement based and consequently has enabled us to exploit natural abstractions found within use case modelling. By underpinning informal use case modelling with Event-B, we are able to provide greater precision and formal assurance when reasoning about concerns identified by safety engineers as well as the subsequent changes made at the level of use case modelling. To achieve this we have extended use case modelling to include the notion of an accident case. Our approach is currently being implemented, and we have an initial prototype.