Senior Machine Learning Scientist

Senior Machine Learning Scientist

UK or Poland (Remote or Hybrid, it's up to you!)

Dependent on experience

Be part of our success with the opportunity to join our company equity scheme

Our Perks

Monthly Health & Wellness budget, increasing with length of service

Annual Learning and Development budget, increasing with length of service

Flexible working in a choice-first environment - we trust the way you want to work!

Work From Home Allowance

25 Holiday Days + your local bank holidays, plus an extra day for every year of service

Your birthday off

Enhanced Family Leave (UK Only), Fertility Leave, and Neonatal Leave

Optional Healthcare Plan

Life & income protection (Location dependent)

Employee Assistance Programme (UK Only)

The opportunity to share in the company's success through options

If you're in London, a dog-friendly office with great classes, events, and a rooftop terrace

The Role

Our mission is to help large successful brands like Uber, Amazon, Wise, HelloFresh (and more!) put their customers at the centre of everything they do. Using best-in-class tech in a fast-developing AI space, our Customer Experience Intelligence platform continuously analyses explicit and implicit feedback to enable our clients to identify what they should do next.

We're hiring a Senior Machine Learning Scientist to join the team and help build and ship the next generation of that stack.

What you'll be doing:

Unlike many companies, we use our own custom models, specialised for customer feedback, across various parts of the stack: extraction, retrieval, reranking, summarisation, and sentiment analysis. We are also pragmatic and understand that the right solution can be a combination of off-the-shelf LLMs, bespoke fine-tuned models, and sometimes techniques that utilise no LLM at all.

This means you will:

• Train, evaluate, and iterate on ML models and agentic systems for customer feedback, including owning our custom fine-tuning pipelines. Run experiments end-to-end, track results rigorously, and make clear recommendations on what to ship, iterate, or retire.
• Build and maintain LLM-powered features: retrieval pipelines, reranking systems, insight agents, data mining agents, and automated taxonomy generation.
• Design and run robust evaluation frameworks: build test sets, define metrics, evaluate non-deterministic systems, handle class imbalance, and automate checkpoint comparisons.
• Improve and extend semantic search and retrieval, evolving from embedding-based approaches toward more advanced methods.
• Write production-quality code and collaborate closely with Engineering on productionisation, model serving, data pipelines, and monitoring.
• Work with Product and Commercial teams to translate business needs into practical ML solutions, and support client evaluations and accuracy benchmarking.
• Mentor team members, review code and research, and bring relevant advances from the literature into the product.

What you'll need:

• A deep working knowledge of transformer architectures.
• Strong PyTorch skills, with the ability to write custom training loops, modify model architectures, and debug issues at the tensor level. Ideally, experience with parameter-efficient fine-tuning techniques such as LoRA
• Extensive experience working with large-scale, messy real-world text data, including classification, extraction, embeddings, re-rankers, clustering, and search.
• Experience in instruction fine-tuning and serving language models, familiarity with frameworks such as vLLM, DeepSpeed, or similar tools
• A solid grounding in classical ML and statistics, and the judgement to choose simpler methods when they're the right solution.
• Practical experience building with GenAI and agentic patterns.
• Excellent communication skills and confidence translating complex technical concepts for non-technical audiences (and vice versa!).
• Technical curiosity and a keen interest in AI – a love of experimenting to make the most of available technology.
• High ownership and initiative, with the ability to identify problems, prioritise effectively, and drive solutions forward.

It would be a bonus if you:

• MSc/PhD in Computer Science, Machine Learning, Artificial Intelligence, Data Science, Computational Linguistics or a closely related STEM field.
• Experience with reinforcement learning techniques, such as with verifiable reward (RLVR)

Our Hiring Process

1. Let's introduce ourselves – you'll complete an introductory asynchronous interview - we'd love to learn more about you, your ambitions, and what you're looking for in your next step.
2. Get to know your would-be manager – you'll have a call with Aji, our Chief Scientist, to learn more about the role and show off your experience.
3. Show us how you work – you'll complete a short take home assignment
4. Get to know your would-be team – You'll meet a mix of people who you'll be working closely with from the Data Science, Engineering and Product teams.
5. How our values and your career goals align – you'll have a call with our cofounder to learn more about life at Chattermill and ensure we're the right place for your next stage of growth.

Our Values

• We are obsessed with experience – We take our mission to rid the world of bad Customer Experience seriously, and we practice what we preach.
• We believe in the power of trust – Whether it's with each other, our customers, partners, or other stakeholders, we always communicate with openness and trust.
• We act as responsible owners – Whether it's about the company, a team, a project, or a task, having the freedom to make decisions in our area of responsibility is a crucial driver for us.
• We share a passion for growth & progress – On every level, we're motivated by taking on new challenges – even if they seem out of reach. We recognise that we are learning machines and we always seek to action feedback and improve collectively.
• We set our ambitions high but stay humble – We've come together to build a product and a category that's never been seen before. While we're an ambitious bunch with lofty goals, we don't approach this goal carelessly.
• We believe the right team is the key to success – At Chattermill we've learned that all our important achievements have been the result of the right people collaborating together – that's why we need you to apply today!

Diversity & Inclusion

We want to enable exceptional experiences for everyone, and to achieve this we need everyone's voice in our team. We are on a mission to bring more diversity into the business and to give everyone (from all backgrounds and abilities) a chance to join us, even if they may not fit all of the requirements set out in this job spec. We realise that some may be hesitant to apply for a role when they don't meet 100% of the listed requirements – we believe in potential and will happily consider all applications based on the skills and experience you have, we'd love to be part of your growth and we encourage you to apply!