Senior Machine Learning Engineer

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

🦸‍♀️ 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 Engineer 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 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