Senior Software Engineer

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 ONSITE This 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 Responsibilities • Lead the investigation and resolution of complex software defects and live system incidents • Perform structured root cause analysis and implement robust, long term fixes rather than short term patches • Deliver refactoring and modernisation work across large, complex codebases • Work across multiple languages and integration layers including Python, C and C++ and where relevant MATLAB and Simulink • Support architecture evolution as system requirements change over time • Deliver performance improvements and optimise existing systems where required • Drive improvements in code quality, maintainability and engineering standards • Mentor and support other engineers through coaching, code review and technical guidance • Contribute to peer review processes and ensure strong quality gates are consistently applied • Provide clear technical input into planning, estimation and delivery acceptance criteria • Produce technical documentation and evidence to support delivery assurance and compliance requirements About You • Strong background as a senior software engineer working on complex, integrated systems • Proven experience in debugging, root cause analysis and resolving high impact technical issues • Strong hands on experience in Python and C or C++ within production environments • Exposure to MATLAB or Simulink integration would be advantageous • Experience working in secure or regulated environments, ideally defence or government • Strong communication skills with the ability to influence engineering standards and mentor others • Comfortable working in legacy codebases while supporting modernisation and refactoring initiatives • Active DV clearance is essential for this role