MERITUS are recruiting for a Senior Software Engineer (SFIA 5) to join a leading Central Government defence organisation, supporting the development, maintenance, and modernisation of complex mission critical software systems within a highly secure engineering environment.SENIOR SOFTWARE ENGINEER (SFIA 5) - PORTSMOUTH - PERMANENT - UP TO £90,000 - ACTIVE DV CLEARANCE REQUIRED - 3 DAYS ONSITEThis role sits at the centre of a technically complex software estate, where reliability, performance, and long term maintainability are critical. You will be working hands on across legacy and modern systems, driving improvement in code quality, resolving high severity defects, and supporting the evolution of engineering standards across the team. The environment is highly collaborative, but expects senior engineers to take ownership of difficult technical problems and lead by example in both delivery and engineering discipline.Key ResponsibilitiesLead the investigation and resolution of complex software defects and live system incidentsPerform structured root cause analysis and implement robust, long term fixes rather than short term patchesDeliver refactoring and modernisation work across large, complex codebasesWork across multiple languages and integration layers including Python, C and C++ and where relevant MATLAB and SimulinkSupport architecture evolution as system requirements change over timeDeliver performance improvements and optimise existing systems where requiredDrive ..... full job details .....