Principal Software Engineer - (.Net, PHP, Umbraco, Drupal) in London

A Principal Engineer is the most senior technical role in manifesto's engineering team, providing leadership when it matters most: whether shaping a new client opportunity, designing complex architecture or resolving a critical live issue. You will work across our client estate rather than being embedded in a single project, moving between platforms and codebases as the work demands. You will hold technical authority across our core stacks, .NET and Umbraco, PHP and Drupal, alongside the cloud infrastructure and hosting platforms that sit underneath them.

The role has three sides to it:

• Shape technical solutions through discovery, technical qualification and solution design.
• Steer delivery on our most complex projects, setting the architectural direction and lifting the standard of the engineering around you.
• Drop into an unfamiliar codebase under pressure and diagnose what is actually happening, giving both our teams and our clients confidence at the moments that matter most.

We build digital products for charities, non-profits and membership and visitor organisations. Our clients are purpose-led, often working with constrained budgets, and the platforms we build carry donation journeys, supporter data and services people depend on. Technical decisions here have real consequences, and this role exists to make sure we get them right.

Responsibilities

• Provide technical authority across our client estate, spanning multiple platforms, hosting environments and codebases.
• Shape technical solutions for new business, from initial qualification through discovery to solution design and estimation.
• Lead the architecture of our largest and most complex client projects.
• Act as the senior technical escalation point for live incidents and complex problems, across multiple stacks.
• Raise the standard of engineering across the department through mentorship, technical review and setting clear standards.
• Build client confidence by representing us credibly with senior technical stakeholders.
• Set and hold the technical direction on our most complex work.
• Define architecture that is appropriate to the client's budget, team and long-term maintenance capability.
• Uphold a clear software design philosophy and ensure teams deliver maintainable solutions.
• Define solutions that are simple by design and challenge requirements that introduce unconsidered complexity.
• Identify and challenge ineffective practices in delivery teams and in client organisations.
• Delegate, and maintain clear separation from work that should be considered routine.
• Make hosting, infrastructure and platform decisions with a clear view of cost, security and who will be responsible for what after launch.
• Diagnose and resolve complex problems in codebases you did not write and may not have seen before.
• Lead the technical response to significant incidents, establishing root cause and directing remediation.
• Identify the systemic causes behind recurring problems and drive the changes that stop them repeating.
• Be an active and visible technical leader across the company.
• Mentor lead and senior engineers, building capability rather than absorbing the hard work yourself.
• Contribute to and uphold our engineering standards, technical governance and software development lifecycle.
• Demonstrate your passion for keeping on top of current tools, standards and trends, including AI-assisted engineering practice.
• Support marketing and new business with prototypes, thought leadership or presentations where it plays to your strengths.
• Create learning within the team by specifying solutions that further our collective knowledge and give others opportunities to grow.

About You

Professional knowledge and experience

• Cross-stack technical depth. Genuine working expertise across more than one platform and language, specifically .NET and Umbraco alongside PHP and Drupal.
• Architectural leadership. Expert in designing robust, scalable and secure architectures for content-managed platforms, integrations and modern web applications.
• Cloud and infrastructure. Practical, hands-on experience with cloud platforms, particularly Azure, including CI/CD pipelines, infrastructure as code and managed hosting environments.
• Diagnostic ability under pressure. Proven ability to enter an unfamiliar codebase, establish what is happening quickly, and lead resolution.
• Pre-sales and technical discovery. Experience qualifying opportunities, running technical discovery, producing solution designs and estimating work before it is sold.
• Software engineering principles. Mastery of clean code, testing practice, design patterns, security and accessibility standards.
• Leadership and mentorship. Demonstrated experience developing lead and senior engineers, raising standards through others rather than personally absorbing the difficult work.
• Commercial awareness. Understands that technical decisions carry margin, risk and maintenance consequences, and factors this into the solutions proposed.
• Communication. Outstanding ability to explain complex technical concepts to non-technical stakeholders, and to hold client confidence when things are difficult.

Technical skills

Essential

• Strong hands-on experience with content management platforms across more than one stack, specifically Umbraco and .NET, and Drupal and PHP.
• Practical cloud engineering experience, primarily Azure, including CI/CD pipelines, infrastructure as code and environment management.
• Experience across managed hosting platforms such as Azure, Acquia, Umbraco Cloud, WP Engine or Pantheon.
• API design and integration, REST and GraphQL, including third-party systems such as CRM, payment and marketing platforms.
• Security by design and DevSecOps practice, with particular awareness of handling supporter and donor data.
• Accessibility standards and practice, WCAG 2.2 and above.
• Version control, branching strategy and release management at scale across multiple codebases.
• Agile delivery practice, and how to make it work in a client services context.

Desirable

• Headless and decoupled CMS architecture.
• Authentication and identity, including OAuth and SSO.
• Search implementation, including AI-assisted and hybrid search.
• AI-assisted engineering practice, agentic tooling or AI-enabled development workflows.
• Experience of the charity, non-profit or membership sector and its particular constraints.

Behaviours

• You are the person others want on the call when something has gone wrong. You stay calm, establish facts quickly and communicate clearly under pressure.
• You are pragmatic rather than purist. You choose the simplest approach that meets the need and challenge complexity that is not earning its place.
• You proactively define and champion the long-term technical direction, ensuring it aligns with business goals and client reality.
• You actively develop and empower other engineers, sharing knowledge and building capability rather than becoming a single point of dependency.
• You move comfortably between client contexts, quickly getting up to speed while maintaining the focus needed to provide effective technical leadership.
• You engage confidently with stakeholders across the business and in client organisations, translating technical concepts and influencing decisions.
• You resolve conflicting requirements and differing opinions into a plan all parties can support.
• You set high standards for quality, reliability, security and maintainability, and hold others to them constructively.
• You seek out new technologies and approaches that deliver genuine value, and are honest about the ones that do not.
• You embody Manifesto's values, championing purpose-driven work and fostering a positive team culture.

About Us

Manifesto is the digital experience agency for changemakers. We are committed to delivering purposeful and positive impact for people, planet and society, in restless pursuit of a better world. We take pride in creating award-winning digital experiences, products and services that are measurably ethical, inclusive, accessible, sustainable and joyful.

Part of the TPXimpact group - a Certified B Corporation, we are a team of 130 digital product leaders, creative designers, user experience designers, engagement specialists, software engineers and experience strategists. TPXimpact is a digital transformation company, and Certified B Corporation, on a mission to build a future where people, places and the planet are supported to thrive.

We are committed to having a positive impact on the clients and the communities we serve. We actively encourage applications from all genders, ethnicities, disabled people, members of the LGBTQIA+ community, and those from lower socio-economic backgrounds.

Don't meet 100% of the criteria? We still want to hear from you. Please let us know if you require any adjustments during the recruitment process.

Advertising Salary: £65,000 - £75,000 per annum