We present an overview of MedSLT, an Open Source platform for developing limited-domain medical speech translation systems. We focus in particular on the speech understanding architecture, which uses grammar-based language models derived using corpus-based specialisation methods from a single linguistically motivated grammar, and summarise the results of two evaluations which investigate the appropriateness of these design choices. Other sections describe the interlingua and its relationship with the recognition architecture, and the current demo system.