Principal .NET Engineer

We’re working with a high-volume technology environment where reliability, observability and engineering quality matter. This is a senior technical role for someone who can set the engineering standard, challenge assumptions and still get hands-on when the problem demands it. Role Overview Location: Remote within the UKPackage: Outside IR35 day rateContract: ASAP start until December 2026Industry: Technology / High-Volume Sports Platforms What You’ll Be Doing Act as the technical authority within a small, highly capable .NET engineering team.Split your time between architecture, technical design, code review and hands-on engineering.Review existing .NET services, test harnesses and validation tooling, quickly forming an evidence-backed view on whether they work as intended.Tackle complex reliability problems across a platform processing millions of messages, where identifying a tiny number of incorrectly handled events is critical.Design meaningful monitoring around data integrity and end-to-end behaviour rather than relying on traditional infrastructure metrics alone.Improve detection of messages that are incorrectly dropped or handled as they move through consumers and downstream systems.Develop smarter approaches to identifying configuration drift between environments while separating legitimate differences from genuine risks.Evolve the existing test harness, extending coverage and introducing realistic mocks for external feed providers and failure scenarios.Help shape the longer-term testing architecture, including where ingress, mocking and validation should sit across the pipeline.Review internal and third-party code critically, challenging technical decisions where the evidence shows something is wrong.Set engineering standards and create a technical culture that helps the wider team become stronger around you.Use AI-assisted development tools intelligently without outsourcing technical judgement or code ownership to them. Main Skills Needed Deep, current commercial experience with .NET, including the ability to review and challenge production-grade code immediately.Previous experience operating as a Lead Engineer, Lead Developer or Principal Engineer, owning the technical output of a delivery team.Strong architecture and system-design capability while remaining comfortable writing and debugging code yourself.Excellent code-review skills with the confidence to challenge internal teams, suppliers or vendors when implementations do not meet the required standard.Strong understanding of testing strategy across distributed systems, including test harnesses, mocks, stubs, failure scenarios and end-to-end validation.Ability to reason about complex systems and identify the right ingress, validation and observability points.Strong analytical thinking, particularly when investigating low-frequency failures hidden within very high volumes of successful processing.Comfortable working with AI coding tools while maintaining complete understanding and ownership of everything committed.A track record of raising engineering standards and developing the technical capability of the people around you.Messaging, streaming, sports betting and specific monitoring-tool experience are not required — strong engineering judgement matters more than domain familiarity. What’s in It for You Outside IR35 engagement.A genuinely technical leadership position rather than a people-management role dressed up as engineering.Complex platform and reliability problems where the answers are not already sitting in a backlog waiting to be implemented.Significant influence over architecture, testing strategy, observability and engineering standards.The opportunity to shape a small technical team and establish the foundations for how it grows.An environment that values evidence, technical challenge and getting to the correct answer rather than simply the quickest one. Fancy a proper challenge? You might have just found it. We are an equal opportunity employer and value diversity at our company. We do not discriminate on the basis of race, religion, colour, national origin, gender, sexual orientation, age, marital status, veteran status, or disability status. By applying you are confirming you are happy to be added to the Addition Solutions mailing list regarding future suitable positions. You can opt out of this at any time simply by contacting one of our consultants.