Senior .NET Developer- Various locations across the UK

FDM is a global business and technology consultancy seeking a Senior .Net Developer to work for our client in the public sector. This is initially a 2-month contract with very good prospects to extend and will be a hybrid role based in one of the various offices in the UK including London, Manchester, Liverpool, Leeds and Newcastle.

Our client is seeking an experienced Senior Developer to provide critical support and resilience for a legacy .NET application that underpins essential safety processes. This system plays a vital role in preventing construction near major hazard sites, reducing risk to the public. Currently maintained by a single developer, the application requires additional expertise to ensure stability, address technical debt, and prepare for future infrastructure migration.

Responsibilities:

• Take ownership of a legacy .NET application, ensuring its continued operation and resilience.
• Diagnose and resolve issues across application, service, and database layers.
• Review and refactor existing C# code and Windows Services to modern standards.
• Plan and execute migration of application components from outdated infrastructure (Windows Server 2008, SQL Server 2000) to modern environments.
• Support potential hosting migration between infrastructure providers in early 2026.
• Collaborate with internal teams to document architecture, risks, and upgrade strategies.
• Provide knowledge transfer and clear documentation for ongoing maintenance.

• .NET Framework & C# Development: Minimum 5 years’ experience with legacy .NET applications.
• Infrastructure Migration: Proven experience upgrading applications from Windows Server 2008/2012 to newer versions or cloud environments.
• Database Expertise: Strong SQL Server skills, including migration planning and performance tuning.
• Configuration Management: Familiarity with IIS and application configuration files (Web.config, .exe.config).
• Windows Services: Experience refactoring legacy services to modern patterns.

We are a business and technology consultancy and one of the UK's leading employers, recruiting the brightest talent to become the innovators of tomorrow. We have centres across Europe, North America and Asia-Pacific, and a global workforce of over 3,500 Consultants. FDM has shown exponential growth throughout the years, firmly establishing itself as an award-winning employer and is listed on the FTSE4Good Index. 

Diversity and Inclusion  

FDM Group is an equal opportunity employer, and all qualified applicants will receive consideration for employment without regard to race, colour, religion, sex, sexual orientation, national origin, age, disability, veteran status or any other status protected by federal, provincial or local laws.

Why join us

• Career coaching, mentoring and access to upskilling throughout your entire FDM career
• Assignments with global companies and opportunities to work abroad
• Opportunity to re-skill and up-skill into new areas, develop non-linear career paths and build a skillset within your field
• Annual leave and work-place pension