Senior PHP Full Stack Software Engineer - Sustainability

We re seeking a UK-based Senior Full Stack Engineer to take technical ownership across our sustainability and digital learning platforms.

You ll be a hands‑on technical leader who takes pride in architecting scalable, high‑quality solutions, enjoys mentoring and raising engineering standards, and is motivated by owning platforms that deliver real impact at scale.

Senior Full Stack Engineer
Old Street, London / Hybrid Working / 1 day a week in the office
£70,000 - £80,000
Plus fantastic benefits: 8% employer pension contribution, mental health support, fitness allowance, enhanced maternity/paternity pay, volunteer days, and more
Permanent - Full Time

Product / Service:
Sustainability Consultancy;
Training & Events
Culture:
Close knit, small and agile company, flat hierarchy, direct communication with director and hiring managers alike

Action Sustainability is a leading consultancy with the aim of inspiring sustainable business.

Our team comprises 60+ full and part‑time employees and associates. Many of our clients are sector leaders on sustainability and supply chain issues, and we are proud of our reputation for leading the thinking in this area.

Action Sustainability drives lasting change in sustainability through innovative technology, expert advisory, and award‑winning learning solutions. They partner with leading organisations, including the NHS and Houses of Parliament, to create measurable impact across industries. As the delivery team behind the Supply Chain Sustainability School, we lead global collaboration to accelerate sustainability.

An accredited Living Wage employer, we ve been recognised by Best Companies with a 'One to Watch' status for their commitment to an outstanding workplace. We prioritise regular wellbeing check‑ins, ensure employees are heard, and offer mental health support. Our competency framework enables employees to shape their career paths with clear development goals, while managers receive coaching, mentoring, and peer‑sharing opportunities to enhance their leadership skills.

This comprehensive approach supports an environment where personal and professional growth thrive.

This is a hands‑on role combining high‑quality delivery with technical leadership designing scalable solutions, improving developer practices, and raising engineering standards while enhancing the learning experience for thousands of users.

We make extensive use of AI‑assisted development tools and want to expand this further so you ll help define best practice for AI‑enabled engineering across the team.

• Lead the design, build and maintenance of custom Moodle plugins, themes and core feature enhancements
• Write high‑quality, secure, reusable PHP and deliver robust front‑end experiences
• Architect and implement RESTful APIs, integrations, and reliable data flows between systems
• Own and drive code review quality, engineering standards, and best practices
• Mentor and support other developers (pairing, coaching, unblocking, raising capability)
• Take accountability for software quality: testing strategy, maintainability, performance, and scalability
• Write and maintain unit tests and Behat tests, and ensure coverage is meaningful

• Diagnose complex issues across application + infrastructure layers and drive resolution
• Lead performance optimisation and scalability improvements
• Ensure compliance with accessibility standards, GDPR, and data protection by design
• Strengthen QA processes and data integrity (including release readiness)

• Work closely with product and design to shape solutions (not just implement tickets)
• Translate stakeholder needs (Learning Technologists, Sustainability Consultants, Designers) into technical plans
• Communicate trade‑offs clearly and proactively manage technical risk
• Participate in (and help improve) agile delivery cycles: estimation, planning, retros, continuous improvement

• Stay current with Moodle ecosystem changes and recommend upgrades/approaches
• Drive improvements to developer experience (tooling, CI/CD, environments, documentation)
• Help define…