This study provides a snapshot of the scale of legacy research waste in the UK. It assesses the current publication status of 145 clinical trials sponsored by ten major UK non-commercial sponsors that were completed or terminated in 2017. Following outreach to sponsors and short-term follow-up, 116/145 trials (80%) had fully reported results, and 11/145 trials (8%) had reported results in the grey literature. Results for 18/145 trials (12%) that enrolled 637 people remained completely unpublished as of early March 2023. Sponsors indicated that they plan to make public the results of 14/18 unreported trials. Our study had an impact on accelerating the reporting of some results, and seems likely to lead to future reductions in research waste. We propose three changes to UK Health Research Authority policies that could improve clinical trial reporting.