Job Description
What you’ll do
We’re creating something special. We are joining forces to create the biggest mobile network provider in the UK. Through our collective strengths and talents, we will transform customers’ experience and create a best‑in‑class network for coverage, connectivity, and reliability, delivering greater value and more choice for the nation than ever before.
There are very few moments in our careers where we can feel the ground shifting beneath us, where we can sense that something bigger than we’ve known before is taking shape. We’re not just building a company. We’re going to build a culture, a legacy and a future to be proud of. This will require remarkable leadership and together we can set the standard in the industry and beyond for both customer and employee experience, in what it is a career‑defining opportunity.
Within this context, the CRO Developer plays a foundational role in supporting the organisation’s conversion rate optimisation agenda. Working alongside the CRO Manager, the Specialist helps design, deliver and analyse digital experiments across all brands, lines of business and customer journeys.
This role contributes to the optimisation programme by helping shape hypotheses, build test variants, managing test setups and supporting with the QA and technical delivery of all testing. It offers a strong development path into a more senior CRO, UX or digital performance career.
Key Responsibilities
* Support the CRO Managers in all aspects of technical feasibility, sizing, prioritisation and delivery across the CRO, future capability and personalisation roadmaps.
* Work closely with UX, Design, Product and Engineering teams to develop test‑ready assets and experiences.
* Build, configure and ensure rigorous QA approvals for experiments across CRO platforms.
* Utilise HTML, CSS and Javascript to execute error free CRO experiments ensuring correct audiences, code, triggers, targeting and measurement rules.
* Monitor experiments in‑flight, checking sample distribution, tracking integrity, audience behaviour and any issues impacting validity.
* Maintain technical aspects of documentation and ticketing including requirements, test logs and results repositories.
* Contribute to a culture of experimentation, encouraging continuous improvement and helping educate teams on best practice.
* Train, guide and support Engineering Devs on CRO best practice.
* Collaborate with both technical and non-technical stakeholders to ensure effective and error free delivery of tests and tracking.
Qualifications
* Experienced in digital optimisation through CRO programmes.
* Familiarity with CRO and experimentation tools (e.g., Optimizely, Adobe Target, Google Optimize, Launch Darkly) — hands‑on experience highly desirable.
* Hands‑on experience with HTML, CSS, Javascript and feature flags to execute best practice error free end‑to‑end front and full stack CRO activities.
* Basic understanding of statistical significance and key digital performance metrics.
* Comfortable collaborating with UX, Design and Engineering teams to shape and build test variants.
* Strong attention to detail with the ability to manage multiple small experiments simultaneously.
* Good written and verbal communication skills, able to work with engineers and non-technical colleagues.
* Strong organisational skills with a proactive, curious mindset and willingness to learn more advanced CRO techniques.
Worried that you don’t meet all the desired criteria exactly?
We know that everyone is unique, with multiple aspects to their identity and different experiences behind them. We are passionate about Inclusion for All and creating a workplace where everyone can thrive, whatever their personal or professional background. If you’re excited about this role but your experience doesn’t align exactly with every part of the job description, we encourage you to apply as you may be the right candidate for this role or another role, and our recruitment team can help you see how your skills fit in.
What we offer
We care about our people’s success by offering great pay, bonuses, up to 28 days off plus bank holidays, and paid time for charity work. You can personalise our benefits for you and your family, like discounts, vouchers, a pension plan and loads more. We help with your career through our amazing learning tools and top‑notch parental leave policies.
Need to know
We are regulated by the Financial Conduct Authority and all offers of employment for this role are subject to background checks, including criminal (DBS) and financial checks to meet the regulator's standards.
We believe everyone should have the opportunity to interview for a role that matches their skills. In collaboration with our Talent, Diversity & Inclusion teams and our employee‑led DEI Networks, we identified a range of reasonable adjustments to help you feel comfortable and perform at your best self during the interview process. If you require any reasonable adjustments or have an accessibility request as part of your recruitment journey, for example, extended time or breaks in between online assessments, a sign language interpreter, or assistive technology, please contact your recruiter directly or email jobs@three.co.uk for guidance.
We use AI in different parts of our business to boost innovation, improve efficiency, and create new opportunities. We know many candidates use AI to fine‑tune their CVs or prepare for interviews, but what we really care about is your unique experiences and achievements.
During the interview, we want you to rely on your own knowledge and skills to show us who you really are—your personality, creativity, and abilities. Above all, we’re looking for authenticity and can’t wait to get to know the real you.
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