The Defence sector is now embracing many new technologies, including Autonomy and Model-Based Engineering. The fast-paced nature of a recent, state-of-the-art eXtra Large Autonomous Underwater Vehicle (XLAUV) engineering demonstrator programme is a case point as it was designed from a Model-Based Engineering perspective. For the assurance case of that programme, it was agreed the best way forward would be to piggy-back on that model-based approach but with a clear structure to assure the Autonomous System aspects. The concepts of Model-Based Safety Assurance (MBSA), and the Safety Assurance of autonomous systems in Complex Environments (SACE)methodology, a recent innovative approach developed by the Centre for Assuring Autonomy (CfAA), appeared to meet the team’s needs. The authors, although aware of these concepts, struggled to find evidence of their use on real-world scenarios, particularly in the Defence sector. So, accepting that a degree of planning and definition of the implementation of the SACE technique would be required, they set about applying MBSA and SACE to their XLAUV engineering demonstrator. This paper provides a summary of the product, and the two methodologies used to assure it. It then focuses on the real-world implementation, highlighting the challenges and benefits for the Safety-Critical Systems community as Autonomous Systems complexity grows, and we continue to grapple with the assurance of them.