Chronic insomnia is undertreated in the UK despite being a common mental disorder that severely affects quality of life. The lead author, a psychiatry trainee, implemented a new group cognitive–behavioural therapy for insomnia (CBT-I) service for secondary care patients in London with chronic insomnia and comorbid mental illness. Expertise was propagated by trainees teaching other trainees. Nine patients completed all sessions, all with moderate-to-severe insomnia on the Insomnia Severity Index (ISI) at baseline assessment (mean score 21.6). All patients seen at follow-up had improved, scoring in the ‘subthreshold’ or ‘no clinically significant insomnia’ ranges on the ISI (mean 6.6), and all with improvements in comorbid psychiatric symptoms and functioning. This evaluation demonstrates that group CBT-I can be easily learned and delivered by those without formal CBT or sleep medicine training. This could increase the availability and accessibility of treatment. However, bureaucratic challenges were faced, and trainee-led innovations should be better facilitated.