Senior Software Engineer in London

We are seeking a Senior Software Engineer who thrives on untangling complex systems and modernising core infrastructure without breaking production. This is an exciting opportunity to modernise core C#/SQL systems!

Impact and Responsibilities:

• Extract legacy business logic, build clean services, and verify changes in shadow mode with zero downtime to revenue flows.
• Extract business rules scattered across C#, embedded JavaScript and undocumented SQL stored procedures into a clean, tested, dependency-free rules assemblies.
• Build new services cleanly, with proper Dependency Injection (DI), robust authentication/authorization, structured logging, and full observability.
• Work fluently across both legacy .NET Framework 4.8 (ASP.NET MVC5, WebAPI 2, EF6) and modern .NET, as both will coexist for the whole transition period.
• Navigate across the multiple existing systems such as the API façade, the ETL pipeline, the SNS/SQS event bus, since neither system is self-sufficient and most real bugs live in the coupling, not one side.
• Deploy changes using shadow-mode verification (compute, log, and compare against legacy outcomes without writing) until equivalence is rigorously proven.
• Refactor and optimise heavy SQL Server stored procedures and backend logic with total precision ensuring client-money correctness is never compromised.

What we're looking for in you:

• Senior-level, hands-on experience in modern .NET (.NET 6+) paired with deep, comfortable familiarity with legacy .NET Framework 4.8 (ASP.NET MVC 5, WebAPI 2, Entity Framework 6).
• High proficiency in reading, understanding, and safely refactoring complex, stored-procedure-heavy database logic.
• Working proficiency in JavaScript/TypeScript, interacting with both jQuery on the legacy .NET front end and modern TypeScript microservices on our sister platform.
• A strong 'legacy technology' instinct over a 'rewrite everything' approach, favouring stability, predictability, and incremental progress over shiny trends.
• Experience or comfort with golden-master testing, diffing, and shadow verification to mathematically/logically prove equivalence to legacy outputs.
• The ability to hold two disparate architectures in your head at once and reason through their edge cases, data flows, and coupling points.

Nice-to-Haves:

• Hands-on experience with AWS Lambda, SQS, SNS, and RDS.
• Comfort operating alongside an in-house AI-agent orchestrator that supports and automates parts of our SDLC.

Our Promise: Shieldpay is an equal opportunities employer. For Shieldpay, building a fair and transparent workforce begins with the recruitment process that does not discriminate on the grounds of gender, sexual orientation, pregnancy or maternity, gender reassignment, race, colour, nationality, ethnic or national origin, religion or belief, disability or age. We offer flexible working options, such as flexible hours and hybrid work, to support our employees' work-life balance.