Neural reranking for UK statutory retrieval: Provision-level evaluation and an open distilled model

Abstract This work explores provision-level retrieval and neural reranking for UK primary and secondary legislation. We introduce UK-StatuteCorpus , a corpus of recent UK Acts and statutory instruments from legislation.gov.uk , together with a 100-query evaluation set of practitioner-style questions whose graded relevance judgements distinguish legally operative, supporting and contextual provisions. Using BM25 and an MPNet-based dense retriever to build candidate sets, we evaluate ten neural rerankers, including transformer cross-encoders, a late-interaction reranker, an LLM-based listwise reranker and proprietary APIs. Across both sparse and dense pools, neural reranking consistently improves normalized Discounted Cumulative Gain (nDCG) and Mean Reciprocal Rank (MRR) over first-stage retrieval. We further distil a proprietary Voyage reranker into a ModernBERT-based cross-encoder, Distilled-Voyage-ModernBERT, which approaches the teacher’s effectiveness and outperforms other open rerankers on our benchmark. Results are based on 100 expert-validated queries, each linked to three graded provisions from a single UK instrument, so they characterise single-instrument, provision-level retrieval over recent UK legislation.